I am a little confused, please bear with me. The pain you describe is low back and abdominal, not kidney? It is possible that it a uti or chronic cystitis, but if the issue was mainly kidney the pain would be
in a different area.
Do you have any fever? Does infection show up on the urine dipsticks they take at the doctor's office?
Do you have any scarring or kidney stones in your kidneys?
You can have infection that does not one standard tests. This type of infection can be several things but one of which is called a biofilm infection. In a biofilm infection that bacteria attached to any foreign objects, like stones or stents or artificial joints etc. Then they basically set up a fortress or in passable city and only slough off in small numbers not have enough in our urine test!!
HOWEVER IN YOUR CASE-I think you may being dealing with either chronic cystitis or Interstitial cystitis. These have very similar symptoms and the only way to know for sure is with treatment of long term antibiotics. A GOOD uro is trained to know the difference.
The key is to call and ask them on Monday, if they treat chronic uti's or complicated infections. Then at least look into to information on Interstitial Cytitis.
We have a group here on medhelp:
I am also an IC support group leader for MOARK IC. You can pm me for more information on IC or Chronic Cystitis.