Bleeding after Mastubating

So I wanted to relieve some sexual tension and masturbate using my vibrator. However, I didn't finish because I couldn't get off and about 30 minutes to an hour later I went to the bathroom and noticed blood on my underwear; now mind you that I just ended my period (but two weeks before that I had unprotected sex which led me to take Plan B as a back up), so I know it's not that why, plus I had no cramps or anything. Can someone help me figure out what's going on?