As an adult I take about a tablespoon of ground flax seed with my cereal everyday, for a baby it should be about a 1/2 teaspoon. It is supposed to help with depression and keeping your mood stable, as well as helping with arthritis, inflammation, and lowering your cholesterol, etc..
You should wait until 8 months to introduce. Because they are a seed and can be an allergen you should talk to your pediatrician first. If you have any family history of nut or seed allergy he will probably tell you to wait until a year.
Could you dd be getting to much iron? Iron is very important, but too much can cause constipation. Just a thought.
other food that help with constipation (treat each as a new food and make sure they are age approprate): apples, carrots, beets, barley, greens, brown rice, asparagus, aloe vera (ask your ped.)
good luck!
how old is you dd? and how is she doing now?