Okay this happens all the time. Sometimes when your body is cleaning out your female parts, your body might "miss some" and be getting rid of older blood after your period. It's nothing to worry about (unless you're sexually active) and don't panic if the blood starts turning brown. Just wear a pad for a couple days and clean yourself frequently throughout the day. Good luck :)
I have had this, it could be one of two things (that I know of.) it could be just spotting before your period, it could have been stopped from stress or eating unhealthy. Or, if you have been sexually active you should ask a doctor if this is normal or not, maybe even take a pregnancy test.