I've been having unprotected sex for a while now, with only one person. My period are never regular, so I don't know what to think. My period was 11 days late, then today, there was a gush of blood. It ruined my panties, and jeans. I put in a regular absorbency tampon, and when I went to change it there was almost nothing on it. Just brown discharge. This all happened within a 12 hour span. Could this have been implantation bleeding?