Curious if I'm pregnant?

Around a month ago me and my boyfriend had unprotected sex and I am pretty sure that he ejaculated in me before he could pull out. I'm 19, almost 20, and although I wasn't planning on having a baby this early, if I turned up pregnant I would actually be happy. About 2 weeks after that, I had a brownish substance coming out that lasted for about a day. Ever since then I have been tired a lot and irritable. Within the last week I've been having nausea and cravings, and my breasts have been extremely tender and seem to have firmed up. I was supposed to have my period a week ago and just today I started spotting brown again.

I have read that having rough sex causes this but when we had sex last it wasn't rough or dry. I took a pregnancy test 2 weeks ago and again earlier this week, both were negative. I'm slightly worried about this. Could I be pregnant??