Urinary Tract infections

I am a 17 year old female and constantly keep getting UTI's. I get one near enough every week with severe pain that has worsened around my lower back and the lower part of my abdomen. No anti-biotics that i have been given work and it is starting to worry me as it is not normal. I have been for a scan on my kidneys and bladder but the result came back normal. When i get a UTI it makes me feel very ill such as feeling sick, tired and also i have the other symptoms of a UTI such as burning and feeling the urge to urinate but when i do go to the toilet nothing comes out. My doctor has just sent me away with pain killers and suggested that it could be my bowel pressing on my kidneys that is why i am getting the pain. I would really like to know though why i keep getting the infections and what else can be done.

Thanks Kelly