Me and my gf had the same problem. What could of happend was he popped your cherry, and if you dont know what that is let me explain. The first few times you have sex the vaginal opening may be tight. Occasionally the opening in the thin tissue (hymen) at the vaginal entrance is not large enough to be stretched easily by the penis. If this tissue tears during sex (ouch!), some people refer to it as “popping the cherry.” The opening can gradually be enlarged during masturbation before having sex. Using lubricated condoms and/or placing several drops of a liquid “personal lubricant” on the condom tip just before having sex often makes sex more comfortable for the woman. The extra lubrication may help the penis stretch the vaginal opening. We trust you know always to use condoms when having sex.
You probably just started your period, this happens to my girlfriend sometimes. If you're very concerned and don't get your period in a few days then maybe see your gyno?