Things that help: Several things may help: a slow release iron or divided the dose to twice a day. Increase your daily fiber intake (shoot for 30 g or more per day). You may also add a mild stool softener (like colace) daily. Some people need to do all of the above. Also, go see the doctor who asked you to take the extra iron and see if he/she can help too.
Increase fluids. Increase your fluids and avoid caffeine and alcohol. There are different formulations of iron so maybe changing to a different form may be tolerated better by you with less constipation. Try a stool softener 1-3 times per day while taking iron and most important be sure you know the underlying reason for your anemia and treat that first!