Constipation from prenatals.

Yall. I take my prenatals daily like it says on the bottle. I been super constipated and it would hurt so bad to poo. But everytime I did, it was like black. So I asked my Dr and he said that it's because my prenatals have iron and my body can't absorb it all. So he told me to switch to prenatals and Flintstone vitamins every other day. Like alternate them. And it's helped so much. Idk if anyone else is having this problem, but if so, maybe this will help