If you feel it is hard to have a bowel movement, like your constipated then it could be iron. Otherwise your 37 weeks, your body, muscles ect are changing/stretching and preparing for birth so it could just be that.
In prenatal pills the iron can do that. Buy fiber gummies and eat broccoli with your next dinner. Should be better in 2days
To much iron. Try green vegetables it should be better in 4 to 5 days.