What's Up With My Snails?

[Raticataticus]

So I got 2 mystery snails free from the pet shop about a week ago. The first few days they seemed really happy, cruising around the tank and climbing up the walls and whatnot. Now they are acting really strange. One got filled with air and kept floating around the tank and whacking up against the sides. Then it deflated itself and went back down to the bottom. Then the other one got a little filled with air too. Then both sat around for a while just in their shells just doing nothing. 
 
I am worried about them dying and fouling up the tank so I have moved them to another little tank by themselves. Now one of them is cruising around and the other it still a little bit filled with air and doing nothing. Neither of them stink and their "doors" seem to be in tact.
 
I will note that all of this happened after I did a big water change.
 
Here is what they looked like when I took them out of the big tank to move them to the small tank.
 

 

[PrairieSunflower]

My assassin snails, when small, seem to climb up the tank and get air bubbles in their shells... none of the older ones ever do it though.