Apple snail has weird organ like protruding?

I’m not sure if this is an emergency. I have two Apple snails that have gotten HUGE since I got them and today I just noticed that one of them has a cream colour globe thingy protruding from under her shell on the opposite side of her siphon. It freaked me out! I have never seen this before in either. She took her siphon out VERY long and far and when she put her siphon back in the cream ballon disappeared into the shell? Is this normal or mantle collapse? Thanks!

Pictures taken in succession:

This is how they take in oxygen. Search on snail anatomy. They have a bladder to store air.

I’ve never seen it out of the shell, the bladder I mean