Hear, I got a kidney inflammation this summer (besides MXE I have used much Desmethyltramadol), all values were fine when checked in hospital (was there for other reasons) but the kidney showed increased bloodflow. They asked me if I had an inflammation there and then I said well, now that you say it, that must have been it! Its characterized by a uncomfortable/annoying/fearful diffuse pain, radiating from your back to the left around your stomach, sort of spasm like.
They said that almost always the cause of this is drinking not sufficiently (and not by cold temp. as old sayings go), yes I might have forgotten to adequally drink when I was , umm, a bit more stoned than usual, almost blacking out. Not a surprise under this condition. Checked my drinking habits closely from then on, lots of cranberries juice which helps as well and never had it again. Either some metabolite or the drug itself might be a bit irritating there or its simply the amount of water missing (or, by backflow of an overful bladder, because of difficulty to pee).
DRINK SUFFICIENTLY WHEN ON MXE! (an btw I never went over 50mg, redosed only twice out of maybe 25 times).