Mid -cycle bleeding

This cycle, my period lasted 9 days, then on cycle day 15 I was bleeding (bright red) for like an hour and then it was gone. Now it's cycle day 18 and I'm bleeding again (this time a darker red almost brown, and I had some clots) My husband and I haven't BD'd because we work opposite schedules and this month has been hectic, so I know it's not implantation bleeding. I've been using OPKs since day 12 and haven't had a positive yet. I also took a pregnancy test (just incase) and it was negative. Any idea on what this could be? I'm calling for an appointment to see a Dr on Monday.