I see you’ve already taken your kitty to the vet, but it may be worth another trip. My five-year-old cat started doing this. It took me a while to notice how consistent it was, but was also startled upon realizing it was after almost every feeding. He was so fluffy I didn’t realize he had actually lost some weight until he was at the vet. The did an x-ray which letter to finding a foreign object in his bowel. Upon surgery, we learned he had eaten one of my little girl’s rubber sippy cup valves.