I encountered this with a sim. The culprit in my case was that this sim kept partaking in food stalls when I played in different households (since this sim was in San Myshuno with the food stalls they would partake in food stalls constantly, especially when I played with other households in that neighborhood.) I had her use her aspirations for the insta-lean potion to remedy this (if I didn't have enough aspiration points for this I would've had to go into CAS and use some cheat codes to fully get her body back down).