She's a very strange mongrel who loves a bum scratch only when you scratch her bum she automatically sticks her nose up her backside and furiously sniffs it while hopping in circles til she falls over. Every time.
Great photo OP. My Lab loves a botty rub. Will reverse into us so we can oblige. Funnily enough, our vet confirmed this by asking if she had a preference for a botty rub or neck rub. It is definitely a dog thing!
Could be skin irritation. Not all dogs are obvious about their glands needing doing. DDog1 will tell you. DDog2 will act like nothing is wrong and you wouldn't know at all.... his are often the fullest and worst. Get her checked