I suggest to start a food diary and then possibly an elimination diet.
Keep track of what she eats VERY specifically, and also write down when her tummy was bloated looking - for a week or 2 (longer if needed) then look back and try to notice if there's any thing that could be a trigger. What she eats today might affect her right away, but it could also be a delayed reaction, so look at what she ate the day before, and 2 days before the bloating happened in the diary.
If this seems too difficult, you could just start the elimination diet and start by eliminating ALL dairy (including sour cream, cream cheese, lactose, casein, whey, etc.- there are so many different names for milk based products. It might not only be a lactose problem, she could be having difficulty with the proteins in milk) and wheat/gluten flours (including oat - anything, bran, barley, semolina, white bread, white flour, anything that says "contains wheat" or "contains gluten", etc.) . Don't give anything that would contain either of these types of foods for at least 3-4 weeks. See what happens.
If you choose to do the elimination diet, there are rice and gluten free breads and mixes that you can make at home which you can use to substitute. Our grocery store carries them in the health food isle. Ask your store manager if you can't find them.
(I have a son who is gluten and dairy intolerant. He has different symptoms than you described, but, what you described is a possible symptom that I have heard from other parents.)
If the bloating doesn't go away with the elimination diet, then it might not be that. Unless you weren't really strict. If it does go away, then you can try adding in ONE of the things only, and see what happens. DO NOT add in both at once! It's critical to be able to determine what the problem is. Then, if the bloating comes back by adding in one, then take her off of it again, stay off both for awhile, maybe a week, then add back in item #2, and see what happens. If the bloating comes back again, then maybe it's both, if it doesn't maybe it's only one.
You'll know more than anyone. AND WRITE DOWN EVERYTHING!!
Keep doing the journal and READ EVERYTHING YOU BUY for ingredients.
I'd HIGHLY recommend that you look at the web site
It's a very thorough resourse of parents who have children with food allergies, or celiac. Sometimes allergy tests are wrong, and sometimes celiac tests are wrong. Check with these VERY knowledgable parents! They have GREAT answers to many questions.