I've had a UTI that spread to my kidneys. This post is going to be to much information, so i apologize now.
I was in college and I didnt' have the time to go to the doctor - I use to get a UTI a month, and had taken care of them myself all of the other times. Cranberry capsules and tons of water. Anyhow, this time, it was severe, i guess and my home remedies didn't work out. Everytime I urinated I peed out something that was the consistency of snot. Full of blood, and more painful then any UTI i've ever had in my life. Not to mention extreme back pain and a fever. I ended up fainting in the hall at school and some one brought me to the ER.
A UTI spread to the kidney's is terrible...and you should be able to know if it is there by the back ache. From your post it doesn't sound that way to me, but it could be different for everybody. Keep on top of those doctors and get yourself better!