Gayle's got it right.
In the short time I've had mine; I've found mine does the same thing. (especially the webbing of my fingers and my rings)
She's grooming you~
Being such social animals; one way they develop relationships and closeness is by grooming one another. (Like monkeys)
Chances are she's trying to groom you to show how much she likes you. :D
If it hurts; as many others have said- go "PSST" at them.
If Ithil continues after that; I'll actually mimic her crabbing sound (though it's much quieter than her's) and usually; it's enough to make her stop.