pregnant?

My bf and I had unprotected sex on the 2nd. This was the 12th day of my cycle. My last period started on nov 20th. I took plan b the next day. Ever since then my back is achy and my boobs hurt. I've taken one pregnancy test last week it came out negative, I don't know if I'm paranoid or what. I'm just scared I'm pregnant and we both ain't ready to be parents. My next period is due on the 19-22.