It could be caused by iron in your prenatal vitamins
. However there are other reasons for this such as chronic asymptomatic bleeding from duodenal or gastric ulcer, it is a very serious problem. You should be examined : full blood count, CEA Blood Test
(Carcinoembryonic Antigen), test for blood in your stool. You have to be also examined per rectum
to exclude cancer. Ultrasound
also should be done because extreme pressure in your vaginal walls can be a symptom of hydramnion.
Wish you and you baby very Good Health
Dr O. Aleksandrov