Bleeding mid cycle

Am
Okay so during my mid cycle, I went to the bathroom at work and when I wiped the tissue had a significant amount of blood on it. None of it got I my underwear and I put a panty liner in and only a little bit got on that but it lasted for my entire 8 hour shift at work and stopped before I got home. I exercise regularly but never had that much blood. Any ideas what this could be from???