Any clue what is going on?

I have been having dark brown discharge each month anywhere from cd 9-13.Sometimes it’s a couple days and sometimes it’s five days.Its mixed with brown bits of tissue,just brown or mucus depending on the day.My periods are regular 28/29 days.Is this low progesterone or estrogen issues? My periods have been pretty normal flow wise but maybe one day less of bleeding so maybe I’m retaining leftover blood and it’s resurfacing before ovulation? Any ideas? Ready to throw in the towel :/

Dark brown is old blood typically.

Yes that's possible and then the estrogen rise/fall aroudn ovulation may stimulate the midcycle bleeding. I have personally had that happen a few times myself over the years.

Another option would be cervical erosion, polyps, cysts, or fibroids. It may be worth having it checked out just to rule that stuff out.