I Had Unprotected Sex January 1st And My Boyfriend Came

I had unprotected sex January 1st and my boyfriend came inside of me. About a week after that I started to have really tender breast, my back was hurting, so I got worried. Then two days later I just randomly got sick and started throwing up. I then took a pregnancy test and it came back negative. I was supposed to start my period the 16th and three days ago I thought I started but it was not like a regular period I have and the pregnancy test still said negative. I only bleed for two days. I still have sore breast, back pain, and feel nauseous a lot but I haven't puked since. What's going on???