Severe constipation?

My 3 year old holds in her poop. I see her do it. She eats 75% fruit- berries, cherries, watermelon, bananas, apples, kiwi, mango, avocado, plum, grapes. She drinks smoothies made with these things, veggies, and fat-free yogurt. All her bread and crackers are wole grain, she eats 0 fast food or corn syrup, and she doesn't eat dairy besides the yogurt. I give her prune juice to drink every morning, and the last time she pooped was Monday (with a suppository). It's now Saturday, I have given her another suppository and she's holding in her poop again. I have her quarantined until she goes, with an enema if necessary. The fact that she eats more than your average adult, in high-fiber roughage, and still manages to hold it in for a week honestly amazes me. What do I do? Nothing is working. Her stomach is ridiculously swollen. I've taken her to 2 doctors, who say "give her healthy food" and that's it, I guess they look at my hefty girl and figure she must be eating nothing but Mickey D's.

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