My dog's long auto-cunnilingus session.
It's one of her favourite activities.
Maybe she's just itchy; might she have an infection? 
She gets regular checkups with the vet and he's never found an infection. The hairs down there can end up kind of glued to her skin with dried saliva, which I think can itch and pinch her and trigger a bout of licking, but after the first 10 seconds of dealing with the itch, I'm pretty sure she just gets into it and decides to go all the way. She can go at it for several minutes, and looks quite satisfied afterwards.
I keep her hair short with an electric trimmer, including the hair in her neither regions, and she seems to enjoy the vibrations from the trimmer when I'm trimming down there. When she sees me with the trimmer, she rolls onto her back and spreads her legs to give me easy access; her ladybits are the only part of her where she actively cooperates in the trimming, and it's often difficult to do the rest of her since she takes every opportunity to roll over and give me the 'hello sailor' pose.
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