New contraction sensation in anus??

Anyone had ever experienced this sensation where the anus contracts internally?

Normally I have the “autofuck” contraction during sessions but yesterday during my usual session after the usual contraction my inside area of the anus sorta contract and relax by itself. Had this sensation for the first time but I’m not complaining ?

Source: https://www.reddit.com/r/aneros/comments/5brir4/new_contraction_sensation_in_anus/