What Causes Mild Uterus Pain?

ok iv had sex a few times since feb 15 with my bf(unprotected) and now latley iv had a tiny pain in my uterus.constantly.everyday, it started maybe a week ago. I cant be prego. Im in college and my mother still thinks im a vigin.what could it be?? im a19 year old female. Im 5"3. And weigh 145. No medical problems.no allergies that i know of.