Is she young? May be she gets startled without you actually intending on doing it. I find some young piggies want to lick you but get a bit too excited and use their teeth a little but as they get older they just lick you
My piggie Dillon has done this since I got him at 12 weeks and he still does it now at about 6 months! Even at lap time he will decide to put his wee nashers round my fingers, and likes to really get hold of nails....weird!
I am hoping he eventually just stops it. My other pigs have never nibbled unless they need the toilet so not sure why he does it....guessing just different personalities