I took Iron tablets when I was pregnant with twins. I just remember that I had to be sure not to take it with my prenatal.
I did not have a problem with constipation. But I remember I did with my first pregnancy and I think I took milk of magnesium. But boy did I go to the bathroom a ton for like a month after and that was from only drinking it once!
A high fiber diet and a fiber supplement with plenty of water would be a great start to helping you with the hard stools. Not all fruits and vegetables are packed with fiber so be sure and add beans to the mix. Hummus always helped me!