could be numerous things

old blood? spotting?
whes the last time you had unprotected sex? are you on any form of birth control

I'd say if there isn't a whole lot of it it's probably just old blood. I sometimes get it a few days after I've stopped my period too (especially when I only bleed for 2 or 3 days). If it continues for a few days, though, it might be best to go to the doctor unless you just started a new birth control pill and are experiencing break through bleeding.