Trying to get pregnant

So I've been dating my boyfriend for like 2 years now and we have been trying to have a baby for like 2-3 months probably even more so I feel when he shoots inside of me, but out of all the times he has I still haven't managed to get pregnant does anyone know why? There be times too where he does shoot and couple days later   get my period. I went to doctor and told her the same and she prescribed me some pills I have been taking them hopefully they work. Has anyone had the same problem?