About 4 years ago I was diagnosed with irratiable bowel syndrome. I was working a very stressfull job when this happened. Bouts of diaherrea and constipation very common. Oh yeah and a ton of gas. But would be different during my menstraul cycle. Would actual be normal.
Anyway left the job and the ibs settled down, but now about a few days before my period, I would be slightly constipated. Then into the second day of my period I would get the diaherrea, explosive type. Colouring would be anywhere from a golden color to dark and tarry looking. Also had stool tested with negative results. I found that what I ate during that time mainly any chocolate which I craved badly before and during my period would produce the black tarry look. Add any stress to this and it would be worse, (more frequent).
You might want get checked out for ibs, it took 8 months for them to diagnose me and I was severe at the time.