My menstrual cycle has been irregular even when I was on birth control. However last month and this month they have been 29 days. This month one cycle day 1 it came on heavy with a few clots. Cycle day 2 was very light, barely anything and migrated into just spotting. Cycle day 3 I had no bleeding what so ever. Cycle day 4 I had to make a bowel movement and started bleeding again. It was almost watery, and it lasted maybe 15 minutes, then switched to a dark brown discharge. What could cause this. I did take a Pregnancy test and it was negative. I am just curious if I need to worry about other possible reasons that maybe serious which would cause this sort of bleeding.