You might have a recurring UTI which doesn't seem to be clearing up.
Last term, I spent 2 months having a UTI every couple of weeks, I don't know why I suddenly got one, because it wasn't the first time that I'd had sex.
However after a few courses of antibiotics and the Christmas holidays, I haven't had a UTI since. *touchwood*
Coincidentally, it was the same case with my best friend. She had UTI's on and off for a couple of months until January.
Anyway, all I can suggest is go to the GP and get some antibiotics for about up to 7 days until the bacteria are really gone, otherwise the infection will just keep recurring.
Try being really stringent with your hygiene before and after sex, making sure you both have clean hands beforehand, and take a shower/wash yourself afterwards.
Also, drink lots of water and cranberry juice as this helps prevent the bacteria settling in your urinary system.
The GP told me that some girls will simply get a UTI everytime they have sex, and in this case, the GP gives you a tiny does of antibiotics to take after every time you have sex.