What Causes Heavy Bleeding & A Brown Discharge During Menstruation?

I had unprotected sex with my husband on the 1st jan, on the 3rd i started with brown discharge this carried on until the 7th when i then started with heavy bleeding which lasted a week. I wasn't due on my period until the 16th/17th so a little confused, I still haven't started my period but today 23rd i have noticed the brown discharge again but this time its sometimes i pinkish, any idea what all this means?