Food Allergies Treatment

My 17 mo. old has always had diarrhea, from mild to extreme, ever since birth. I stopped breastfeeding at 4 mo., soy formula made things better, but it still was never solid. At 1 yr we attempted slowly switching to milk. After 3 days it was really bad and she was very cranky. So next we tried soy milk with no luck. I thought it was very odd that if the soy formula helped, why not the soy milk? We also tried Lactaid with no luck either. So now she has been on rice milk for about a month or so and viola! Solid bm’s and no more eczema!
The current problem, which is the main part of my question is………she never had issues with yogurt or cheese (or so I thought) but a few days ago she had a few helpings in one day and the diarrhea came roaring back. Is it common for things to keep changing like this? I hate the thought of her missing out on so many foods. (fyi – she has very little juice, so that is not the cause) Any help would be great!