it could be a uti. it could be an std. it could be that your tore. go to the doctors
Is it a burning pain when you urinate?
If so it may be cystitis, which is just inflammation of your bladder (it can often be caused by sex). If you drink plenty of fluids, particularly water (and I've heard cranberry juice is good too) it should pass by itself.
However, if it persists it may lead to a UTI (a bacterial infection), which can easily be cleared up with antibiotics. :)
if it continues to happen, i would see your doctor. did you use protection?