I had a cousin with a similar problem. He thought he was gaining weight and is normally thin/trim, even at his age. Late sixties. No amount of exercise/dieting worked. Ended up having a 25 pound tumor.
I had an Aunt in her forties that went in to the hospital for a tumor removal, but it turned out to be my cousin Charlie.
But, geez, Charlie only weighed about 8 lbs.