What Causes Brown Discharge After Miscarriage?

I had a miscarriage in dec 2010 and a week later had unprotected sex with my husband. I thought that i had a normal period in January but then in february i had brown bloody discharge for two day and again this month. everyone seems to think that i'm pregnant again but i don't want to take a test. is it possible that i could have gotten pregnant after the miscarriage? and if so should i be having brown discharge three months after?