I had the same problem, so much so that I used to get Kidney infections, you don't want one of them.
Basically it turned out I wasnt drinking enough water, i.e. none at all. I had to make sure I drink at least a litre a day and as the above poster said, Cranberry Juice or supplments can prevent/sooth mild cases. Also make sure you go to the loo after sex (not immediately!! just within an hor) and wipe front to back.
I know your pain. Try explaining it to your parents!