I am sorry your daughter is having these symptoms.
Here are a couple of ideas:
It could be the milk as you suggested. It is possible to be able to tolerate milk and later become [wiki]Lactose[/wiki] intolerant. It is also possible she can handle a little milk but a larger quantity may overload her system (I dont know how much she is drinking).
- If she has Celiac disease / Gluten allergy, whether diagnosed or not, she may not be able to handle too much Lactose. If a person has Celiac Disease and is not on a gluten free diet, they may become Lactose intolerant.
Another possibility is that her symptoms are caused by [wiki]Carrageenan[/wiki]. Carrageenan is an ingredient in the "Boost" beverage. Although it is present in small amounts, Carrageenan has been shown to cause symptoms and Gastrointestinal changes that are similar to IBD in laboratory animals.
Boost also has quite a high sugar content. While this may help your daughter to put on weight more easily, it can also feed Gastrointestinal microbes and cause them to overgrow, called [wiki]Microbial Dysbiosis[/wiki], it can increase Crohn's symptoms.
There is also a stomach virus going around right now.
In order to test whether the Boost is causing her symptoms, you can remove it from her diet for a couple of weeks. Hopefully the symptoms will stop. Then, add the Boost back into her diet in the same quantity as you had before. If the symptoms return, it is likely an ingredient in the Boost. If not, it might have been a virus or something else.
Ensure has some product lines that are milk free and Carrageenan free. Check the ingredient labels because they have products which contain milk and Carrageenan.
I hope she feels better soon.