Hey..... My Last Menstural Period Was November 8 And It

Hey..... my last menstural period was November 8 and it lasted to the 13 and the 14 and 15 I had unprotected sex and I know for sure he cummed in me and I also had unprotected sex on the 28th and 29th and the 3 of December I came on but I suppose to been coming on that week after but I didnt come on heavy at all it wasn't a normal period I took a pregnancy test a few days ago and it came out negative and now I'm cramping and I'm wondering whats going on...please help me understand what's going on.