I cant control my pee when on my period?

You can try this: make sure your legs are wide apart when urinating. Stand up slightly and count to 10 then sit back down and relax (breathe in an out deeply a couple of times) and try to urinate again. At your age, what is happening is called vaginal voiding and some of the urine goes back up into the vagina and leaks out. When not on your period, you can also try Kegel exercises (Google that) and do those. While it is true that your uterus enlarges when having your period, at your age that should not be a problem though it could be just how your organs have formed too (nothing wrong with that, just how you are made). Also, when on your period you may notice you have to urinate more often and that's fine. You should be going to urinate every two hours or so to make sure that the bladder is empty most of the time.

Aw hun, there's really nothing you can do. The bladder(in front) is right against the uterus and during menstruation it swells. It creates a weird pressure between the bladder and intestines where it sits. Its basically smothering your bladder(it gets hard). I can say so myself because I can't hold my bladder either.