What Causes Vaginal Bleeding While Having Sex?

My boyfriend and I have had sex about two times before but today when we tried, right when his penis entered my vagina, i started bleeding.After about 5-10 minutes, it stopped and nothing came out. we tried again te second time, and it started bleeding again? I am concerned and confused why I am bleeding because the first few times we had sex, i have never had this problem. we always use a condom and i have no pain anywhere and i dont continue to bleed.