Annoying black discharge

For the past 8 months or so, after every period, I have old blood that comes out for several days/a week after my period actually finishes. It makes it really difficult to pinpoint the end of my period, plus it's annoying because I've ruined several nice panties, thinking my cycle was over and failing to wear a liner. This never used to happen--previously I've always had a clean quick end to my periods. I'm not sexually active, I'm not on any birth control. What causes this? How can I get it to stop? I've tried inserting thin tampons, and rinsing out my vagina with my finger in the shower, but it doesn't help. It's almost like having a second, lighter period of dead blood. Help!