What Causes Brown Vaginal Discharge After Having Sex?

I am 28 years old female i have noticed after my last period i am bleeding and since then after sex i also bleed. But the worst was yesterday when i made love my partner was covered in blood where as before it was just a little bleeding but now it is alot. But today i have had a browny discharge. This all started off with a greeny colour discharge but now is brown. I also get very bad abdominal pains which run through to my back what can this be?