Constipation

LADIES! I'm currently 22 weeks with baby #2, during my first pregnant/birth I lost over 1.8 litres of blood during my daughters birth. 18 months later and iv just been told my iron is severely low, and I need to take iron tablets. BUT the thing they didn't tell me was, how bad of constipation I would get from these tablets. Been in total agony from straining and pushing ( sorry for the detail) for the last 45-50 minutes on the toilet, I could actually cry over how painful this is and how much it's killing my back!!!