You don’t have to actually stop the plaquenil… You are always part of the decisions, as the person expetiencing the ilmess and its treatments, plus the only one who knows every minute your body goes through over your whole life so far, etc. Details your doctors would all be better off knowing, but don’t have the time to hear it all. The nephrologist (kidney doc), didn’t take anywhere near the time to find out not only your comprehensive history, but even your current illness diagnosed by your other docs! He sounds like a pompous idiot, frankly, and should not mess with meds prescribed by your RHEUMIE for a rheumatological condition! Staying on the plaquenil till you can check with your rheumie makes perfect sense and no doctor would give you a hard time about it. Well, I mean any good doctor, of course! Your rheumie might even give you a pat on the back for staying the course and avoiding flare-provoking changes (stopping plaquenil can cause flares… One legit reason to stop it abruptly is if you have liver problems developing, since it is hard on your liver, but otherwise it tends to be wise to stay on it until you and your doctor are fully convinced it’s not needed.)
Disclaimer: this is not to say there might be some other urgent reason the nephrologist found that makes plaquenil harmful to you at this time. Did he describe why to stop it? If you’re confused, you can definitely call him back to ask (and if your confusion is making it difficult, have a family member or friend help take notes/clarify.) “Not thinking you have lupus” isn’t good enough, plus just about every specialization other than rheum thinks you don’t have it! Rheumatologists are pretty much the only ones who can ever (or should ever) make the final call on diagnosing it. A primary provider who suspects it, even strongly, should send you to a rheumie! So should any specialist who finds signs of it, such as lupus nephritis found by a nephrologist. A nephrologist simply doesn’t know enough about the system-wide effects of lupus (which in itself is a central characteristic of the disease!) to comment on whether you “have SLE”, and certainly has no place taking you off meds to contain it. I’m guessing he didn’t see lupus nephritis, but that doesn’t mean you don’t have lupus involvement in every other body system he’s not checking! Plus, changes in activity over time are another major characteristic, so a one-time snapshot is never valid to dismiss a case, and usually not enough to disgnose unless there are tons of clear signs all at once (rare unless it’s gotten you really sick with lots of organ damage, including perhaps from nephritis.) Ok, I’ll get off the soapbox now, but doctors who act like this make me so mad!
I would, however, take his advice on the water pills, since that is actually within his scope of practice. Un-diagnosing lupus or even changing lupus management drugs without checking with your rheumatologist, are way out of line and probably way off the mark. Especially after a 20-minute visit. It would be different if you thought the meds were causing you problems and they OKed you to stop, but you are fully able to keep taking meds they advise against. It is medical advice, not dictation of requirements, and the worst you could be guilty of is going “Against Medical Advice”, AMA noted in your chart, which is basically a form of CYA for them (Cover Your A–)
I’m givimg you my best assessment of what happened and info about lupus as a whole, and especially advocating for patient participation in medical decision-making. An example of CYA: I don’t wanna be guilty of practicing medicine without a license (though that would really only happen if I, say, gave you my own plaquenil to take), so I’ll tell you IF IT WERE ME, I would ignore his advice on plaquenil and follow up with Rheum about it (ASAP, since your health situation is quite acute/serious with an ER visit! They should be able to get you in, and make sure to advocate for yourself by pointing these things out if they try to tell you it will take weeks to get in!) Plaquenil is long-acting and not very drastic in its action, so delaying changes by a week or so won’t matter much at all and will allow your rheumie to assess. I’ve heard enough horror stories on here about missing/stopping plaquenil that I’ve never missed mine more than one day in a row. It’s far harder to control a flare once it’s started than to prevent it (ounce of prevention worth a pound of cure…), which is what makes me say this nephrologist is being totally reckless on this one and putting you at unnecessary risk without adequate investigation and without applicable credentials (Rheum board-cert, perhaps a lupus fellowship too) to make that call.
As for the water pills, it’s more complicated. It sounds like you know what your body does without them, but they are totally related to what your kidneys are doing and may be causing (likely are!) some of your critical symptoms like dehydration and kidney failure (you said kidneys “shut down” but I don’t know exactly what to make of it.) The key point here is that diuretics (water pills) are all about messing with your kidney function and can be very risky for changing your fluids and especially electrolytes! For that reason, I would never go AMA with this type of drug or acute concern! I would call the office to clarify why I should go off the pills, and keep persisting till I got an adequate explanation of what he found that made him want you off them. If he doesn’t have a good reason and is just trying out a standard fix that works for most people with these symptoms, but not necessarily those with complex diseases like SLE, that’s bordering on malpractice, and he better be using CYA language in his notes in case it really does you harm!
So, in summary: I’d stay ON plaquenil and follow up with rheum about the question of whether you have lupus or not (probably a ridiculous question!) and get OFF the diuretics (water pills) exactly as the doctor instructed but still get in touch with him (or a nurse of his) to find out why and express your concerns. A doctor is ALWAYS ethically obligated to explain, in a way you can easily understand, the risks, benefits and reasons of starting or stopping any medication, and you are not at all out of line to ask again when it isn’t clear. Even if he told you at the visit and you were confused or forgot, don’t feel bad about that and just remember that getting the info is the responsible way to manage your health, especially while in the acute (currently problematic) state you’re in! Best of luck, and I hope you’ll also discuss your concerns about the nephrologist’s hastiness with your rheumatologist. Doctors often scold and police each other, for one. But more importantly, your rheumie may be able to give you pointers on why and when others will doubt your condition and how to handle it. Heck, I’d add that specific question of handling other docs to my list if I were in your position! Maybe your rheumie will even roll his/her eyes along with you about the nephrolojerk 
Mine always does when he reassures me to stay the course of managing my lupus and listening to my body. So lucky to have him, but yours may be train-able if not already so helpful 