Usually first day of period is brown blood, then red. past 3 days this period is very clotty and only dark brown, is this decidual bleeding?

"period" ?'s: All true menstrual bleeding involves a breakdown of endometrial lining. Other disease states (miscarriage, fibroids, polyps, etc.) can cause heavier bleeding and clots. Pregnancies outside the uterus (ectopic) can cause this sort of bleeding as well, so see your gyn physician for an exam and testing, and if pregnancy is possible, obtain a serum HCG test to be certain.