What Causes Bleeding Through Vagina After Getting Kicked In The Stomach?

I have been taking Tri-Sprintec Since December of 2010. I missed my period last month, with two home pregnancy tests negative, a test at doctors negative, and blood negative. Me and my young brother were wrestling and i got kicked in the stomach and starting bleeding almost instantly. I have been on my periods for ten days now. It started thick and dark it isnt as thick but is still darks with little blood clots. Any help?