Not a doctor but I know from recent experience that iron supplements can definitely cause GI havoc. Run this by your gastro but if you're confident that the iron is setting off constipation then laxatives may be appropriate. I had some severe constipation about six months ago and initially I found magnesium pills to be pretty effective initially. Eventually that stopped really doing much and then Miralax/Purelax did the trick. Had to be on a LOT of Miralax, like two scoops twice a day, but it kept me moving and my gastro didn't seem concerned with how much I was taking.

Although, perhaps because my constipation got to the point where an x-ray became warranted and, yup, I was apparently COMPLETELY backed up throughout my entire GI tract...