D.Y.
I don't have a solution, but I wanted to share with you that my 14-month old does the same thing! It does not matter what I am wearing. She will figure out a way to get under my clothes to pinch me. My daycare providers (her grandparents) tell me that she pinches on the arm or neck when she's about to fall asleep too. I have noticed that if she is not pinching, she will pull her hair. Maybe it's a self-soothing mechanism?
We're close to weaning, and she only nurses in the morning and at night when I come from work. I've also tried pulling her hand away and telling her that it hurts Mommy, so far no luck. No matter how short I trim her nails, those little fingers sure can pinch!