HI--
This happened to me with my oldest. It turned out he had food allergies. What happens is food allergies put stress on the liver and gall bladder so they don't function correctly. They don't produce enough bile, which is what lubricates the digestive tract, and therefore baby gets constipated. I had to take quite a few things out of my diet (I breast fed as well). Other signs of allergy/sensitivity to foods include eczema, a "target" rash (redness just around the anus), dark and/or puffy circles under the eyes, a persistent cough or runny nose, swollen tonsils (with no signs of sickness), frequent ear infections....I'm forgetting something, but you get the idea.
Also, babies digestive tracts are unable to properly digest grains until around one year of age. The body must produce a particular enzyme to digest those starches, and that enzyme isn't produced until around one year. That is why cereal so often constipates babies. I would take her off the cereal and just do fruits and vegetables. Prunes, flax seeds, etc can help, but it's better to treat the problem versus the symptoms.
I'm a bit of an expert on this after both my boys have dealt with these issues--if you feel like I'm on the right track with your little one, feel free to contact me with more questions.
Good luck!
J.