A woman that used to work for me had a similar problem. Her stomach looked like she was 8 months pregnant. She was very obese at about 300 lbs, and they took a 50 pound non cancerous tumor out of her abdomen. Next time we saw her, she looked like a woman who had been pregnant and had given birth. She had lost almost 80 lbs within a couple of months. Last time I saw her, she was skinny.
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.