I have been taking ferrous fumarate iron for a month and i'm gaining a lot of weight very fast, can be the iron causing this? i eat very healthy and never gained weight so fast. is there any other kind of iron that won't cause weight gain?

Iron supplements in general do not cause weight gain. If you feel that you are gaining weight out of proportion to how you are eating and exercising, you should talk to your doctor to be sure that there is not another cause present. You may want to ask your doctor if you need to have your thyroid checked. Otherwise, eat healthy foods in moderation and exercise. Good luck.