Sorry to hear about your back problem. Obviously, if you think it may be a kidney problem you should be talking to your midwife and/or GP. As far as I am aware a UTI will probably result in frequent trips to the loo (even more than you would expect while being pregnant!) and then wondering why you bothered after you pass next to nothing - which will probably give you a burning sensation at the same time! However, I am not a doctor so if you have any doubts please contact one or your Midwife.
There are other causes of back pain during pregnancy. Probably one of the most common is due to posture. Have you had your posture checked by an expert? I have, of course (given the nature or the website) assumed that you are pregnant. If you are not, the posture issue could still be relevant. Have you tried complementary therapies such as massage or Aromatherapy?
Sorry, but I can't be of much help without knowing more of your background and being able to check your posture. However, I hope this little bit of information is of use to you.