Anemone swallowing all tentacles?

[seashells]

Looked in the tank & all were well. Hour later all that was visable of our Condi was a round white disk with a small hole in it...looked kinda like a sponge. Almosted panicked, but didn't look mushy at all...just no tentacles. Awhile later 1/2 hr or so & tentacles are reapearing & reinflating. Anyone seen this before? Is there a cause, or just a thing that they do?