my LO used to grab some of my skin and "pinch" but not hard as well whilst she was younger, and b/feeding or when i put her to lay down next to me. as it didn't hurt too much, i left it, but i would gently .remove her hands usually and put them at her side or elsewhere.
have you heard of the chinese expression "gun cheurng" ? it's like someone is in such pleasure their whole body tenses or they just shiver with delight? well i put it down to her showing affection or intense pleasure. sounds weird i am sure. it has stopped (she's now 18m, and think it stopped around the 14-15m mark) however she is very touchy towards me, always putting her hand up/down my shirt, up my shorts to feel my skin. she has her hands either rubbing my neck, or my waist generally. sometimes it annoys me physically with all that rubbing and grabbing, but it is sweet that she wants to be close to me.
perhaps, because it hurts, you can pull her hand away, look her in the eye and say "ouch, that hurts mummy. pls don't do that" each time your LO does it. eventually it may get through to them. it will surprise you how they will learn from firm but gentle "nos"
good luck!
