Pregnant?

I'm not sure if I'm over reacting but my period isn't really regular but I had it last month had sex sometime after and started bleeding a little again the next day. I'm not on birth control and when he ejaculated he put it right back in and we had sex again and that was 4 or 5 weeks ago and recently I've been feeling a little nauseas and my stomach is cramping SO BAD it's keeping me up at night and it's harder than usual I just don't know if I'm over reacting or just being sure. Opinions btw I'm 17