What Is The Cause Of A Brown Discharge?

I took one shot of depo back in November. I bled for 3 months straight afterwards. I never returned for a second shot. I finally stopped bleeding two weeks ago (a few days after I started birth control pills). I have had a nasty smelly brown discharge that comes out even more during sex. It is thin...not thick. Is this normal? Could it be something serious like endometriosis? Sincerely, Chelsea