something wrong?

You might want to contact a vet and tell them whats going on. The only thing I could think of would be a foreign object.

Unless a broken rib might stick out like you described? I would still call a vet especially if you notice it again or she starts to refuse food.

Hope she's ok.

If it was her belly, the muscles they use to push and move can sometimes twitch like that, and even more creepy, they can do it down the belly when you hold them in the air looking for purchase. It looks like a bunch of muscle spasms crawling down their belly. (I know that doesnt make much sense. )

It's a strange sight.