bleeding after sex

I'm 18 and have had sex 3 times with my boyfriend. The first time I started bleeding (not like period, more brown & in less quantity) two days after sex & this lasted for about 4 days. The second time I didn't have any problems, and I had sex for the third time one week ago, and now I've started bleeding brown blood and in a small quantity, & have slight cramping. It's not my period, which is only supposed to come in 9 days. I don't think I can be pregnant because we used a condom & he didn't cum inside me... What could it be??