What Causes Abdomen And Back Pain?

I came off my period 5days ago had unprotected sex 4 days ago. Began feeling sick 3 days ago then the sickness continued and I began to get back pain and headache. Then yesterday I started bleeding took a pregnancy test it came back negative. Then this morning I still have all the symptoms I'm still bleeding but on my right side of my lower abdomen there's abit of pain what could this all be ?