I always think Munchausen by Proxy in these cases.
Common enough that it should be checked first.
That it be checked, yes. That such sensitivities are real, I have my own case to a far lesser degree which is clear enough. In my own case, it was not a matter of misdiagnosis or psychological factors, but just better science.
It is not like the allergy tests of old. It is an established sensitivity to certain foods, not by birth, but developed over time and revealed through science.
My generalized symptoms of being run down, and very real angina-like phantom symptoms making it difficult to negotiate a flight of stairs convinced me something was haywire. The doctor was not sure, either, ran a blood test through a very state-of-the-art lab and it was they who discovered the food sensitivities. Just cutting out those foods made a dramatic difference as I noted upthread. Months after modifying my diet, I broke out into very sharp reaction to my surprise. It was only after reexamining my diet that day that I discovered a tiny amount of spinach present in a salad. That was what did it.
Results to me are dramatic. Stubborn weight I could not lose is simply gone. I feel far better now, and if someone slips into my food one of the items to which I am sensitive, I react within an hour.
No Munchausen for this Munchkin...