Some of the soy formulas do have a significant amount of iron in them which may also be upsetting to your child. (home.allergicchild.com)
If you're a vegan, you may need to take a vitamin with iron in it. (mindbodygreen.com)
It's the reason why prenatal vitamins tend to have so much iron in them (which is why some women have tummy troubles with them). (breastfeedbabywearclothdiaper.com)