Hello!
It seems to be a reversion.
Check the Thread I linked below ;-)

Josh

Here is a link that might be useful: Easter Cactus! - Bloated cactus-like leaves?

Thanks, Josh! How strange. I'll just keep watching it and see what it does. That other thread said that these may occur more often when the plant is stressed, which is definitely the case with my plant :(

Schizophrenia. Or multiple personality disorder. Yeah, that's the one. Your cactus is turning into something else in order to escape the trauma of its real life.

fyi -- I do believe that they call the "spring cactus" Easter cactus?

Last year my 'spring' cactus' put out a plethora of lovely light pink flowers -- this year, none. :-(

That is really odd regarding your "mutation" -- makes one wonder if some of these plants aren't "creations" of humans.