Severe pain, acute renal failure

I had a UTI about 4 weeks ago that I treated at home and felt like resolved after 5-6 days. A couple of weeks later I started feeling completely sick/horrible and my back started hurting pretty bad. I thought maybe the UTI had spread to my kidneys so I went to the doc to have labs done. There was a very small amount of bacteria and blood in my urine, but the culture showed no growth and the sonogram of kidneys/bladder were normal so the doc declared no infection anywhere. But my blood work did indeed show acute kidney failure:

BUN 25
Creatinine 1.6
GFR 40

And I have almost every single possible symptom of acute kidney failure as well (fatigue, backache, nausea, massively increased urine output, unquenchable thirst, bloating, swollen ankles, itching, etc).

I see a nephrologist on Monday but I was getting concerned about waiting because the most troublesome symptom of all is malaise. My muscles all hurt and I know it sounds melodramatic but i feel like I'm frikken dying. The intensity waxes and wanes day by day. I spoke with the nephrologist's nurse about waiting until Monday and she of course gave the standard "Go to the ER if you're in significant pain" and went on to say that with kidney failure there's usually not too much pain. Infection usually will cause a lot of pain, but with failure it's not very common.

Does anyone have any ideas about what could be causing the pain? I've of course spent a lot of time googling all the potential root conditions that most commonly cause kidney failure, but I don't really think any of them apply to me. I'm not diabetic, no high blood pressure. But, I have had positive ANA for years and do have a rheumatologist who has mildly suspected Lupus for years, but my positive ANA is a weak positive and I haven't had any rashes so no diagnosis yet. Are there any other potential underlying causes that could be causing the severe pain?