I was recently in a car accident. That day I began what seemed to be a period. The strange thing was the blood was brownish and heavy. I just had a period about a two maybe two and a half weeks prior to the accident, and had unsafe sex after. The cramps were very bad and the "period" lasted maybe 3 full days. I've never experienced something like this and I fear that it was a miscarriage. Can someone help me figure this out...

No one here could possibly know. You need to see a Physician.

This could be a miscarriage or internal injury or anything in between.

Sounds much more like a internal injury from the wreck, It most certainly needs to be looked at my your doctor