If it is more like skin without a lot of substance, it's most likely a primordial pouch. Which is a cat thing where they have loose skin on their belly. Even big cats, most notably cheetas, have them. It's simply a genetic feature.
If it feels like there is something hard in it, you need to see a vet.
If it feels soft a squishy, she might be getting fatter.
It's hard to know though from online. Truly, the best bet is a follow up with your vet to make sure it is nothing serious.