The informant is Chinese-American where both of her parents are from China, but she has never been there herself. She said that her mom had several superstitions that she would tell her as a child. The informant seemed to feel fondly about this habit of her mother’s.
The informant described this superstition that her mother held following the description of a similar superstition that her mother held. In this way, it was among a list of funny superstitions the mother held that made her seem somewhat quirky.
You should not run or exercise, or whatever, after you eat or else your belly button will shrink… and you won’t have a belly button… and you might die or something
The informants mother probably initially performed this superstition for her daughter either because she actually believed it or to warn against doing strenuous exercise immediately after eating. While it likely won’t shrink your belly button and kill you, it can still cause stomach cramps. The informant seemed to perform the piece of folklore somewhat ironically, laughing at the strange repercussions even as she told it. However, it is still something that stuck with her and that she shares with others as a way of exhibiting her fondness for her mother with her friends.