Why??

Every time my period comes around I'm always superrrr late! Last time for 4 months and than after going to the doctors seeing if I was pregnant or sum wrong but they said I was healthy and I may wanna start birth control. Well I finally get my period and I'm so happy than I get another one than I was due 16 days ago and well I haven't started. Me and my boyfriend have sex a lot and never use protection, he always pulls out and pee's before do. I just get so scared when I'm not on it and stress out. What can I do?I'm also 16