@lauracbaker It sounds like the same thing that happened to myself at the age of 30, I could no longer consume dairy products. I went to see my doctor and he simply looked at me and said that I was lactose intolerant, which can happen at any age, but my case was a severe one, even something as a few plain M&Ms would destroy my stomach.
I lived in fear of being "poisoned" by something like a hamburger bun or dinner roll at a restaurant for over a decade before finding an OTC lactose support supplement that actually worked. I took those up until almost a year ago when they stopped working for me, then they were no longer available. Again, no dairy allowed for several months, until I found something else online that lets me eat anything I want to again.
The reason we can no longer tolerate dairy products is because our bodies no longer produce the enzyme that's needed to digest them. But it turns out that there are supplements that are specifically designed to do that, and they're available on Amazon. I hate having to give Jeff Bezos any more of our money, but in this case I'll make an exception. What I've found that lets me eat anything I'd like to again is a Bacillus Coagulans probiotic from nfsupplements. A bottle of 240 tablets is less than $12.00, and I just take one every morning and every night.
Check them out, they should help you as well I'd think.