Sounds very similar to my background!
Certain dairy foods and ingredients destroy me yet I can drink milk, cream and eat cheese until the cows come home.
The big problem I have is when that dairy product is processed or altered like a typical commercially available cheesecake or in the case of some cheeses cooked.
After years of dietary experimentation (following a medical exclusion, keeping diaries and working out what I ate just before running to the toilet) I can only assume it is a protein issue rather than lactose.
If you don’t mind sharing, do you have any other allergy or sensitivity type issues? For example asthma, allergic rhinitis, specific food allergies, or even minor allergic reactions like oral allergy syndrome?
I only ask as I have met a few people over the years who have similar results from eating dairy and every one of them had at least one of the above.
A family member has a genuine food allergy and I spent a lot of time talking with the guys about patterns and likelihood of if you have allergy or intolerance X you are likely to have Y.
It has helped me make some dietary choices and I can genuinely manage any symptoms through avoiding certain foods.