As someone who suffered from constipation issues my entire life--I am now 42--I never knew until I was 40 that I was allergic to milk casein (was breastfed, drank lots and lots of cow's milk, loved cheese, never ever thought my issues could have anything to do with cow's milk), have severe gluten and soy intolerance, and am also intolerant, to various degrees, to 21 other foods. The thing with food intolerances is, if you don't know about them, they will continue to hurt and wreck your body until it gets to a point--usually in middle age--when you get sick or have really weird symptoms and you AND the allopathic doctor have absolutely no idea what's wrong with you.
That's what happened to me. I got sick with 5 different, really weird symptoms/issues and my allopathic doctor couldn't tell me why I was sick or how I got them. So I went to alternative healthcare and found a wonderful naturopathic doctor, who helped me figure out that FOOD was what was causing the vast majority of my issues.
The best thing you could ever do for your daughter is to take her to a naturopathic doctor and have a food intolerance/sensitivty screen run (allopathic doctors will not check for food allergies unless you have been in anapalytic shock). And I got lucky; although I have an allergy to casein in cow's milk, I don't in goat or sheep and so can consume those products.
It is thought that I have had the milk casein allergy since birth and, because it wasn't detected, all the other food intolerances then came about because I developed Leaky Gut. My daughter also has constipation issues and has her entire life (she's 10). I did have her IgA tested, and she has mild gluten intolerance but is not intolerant/sensitive to soy, egg, or milk casein. But I still want to have her tested with a bigger panel.
Good luck.