Look at the "History" tab (not sure what palette you may have it associated to, mine is with the "Action" tab). The History palette stores step-by-step states at the bottom, recording everything you do to your file. Take a look at the options regarding the History feature and the History brush. You can choose "how many" of the most recent changes you want the palette to retain (keep in mind the "RAM"ifications constraints ;) as more "states" put more strain on your RAM)

It's a great feature and is something that i can explain more about in a little while if you're interested.

:)
retrocity