Is my rabbit being affectionate or aggressive when she nips me?

I've recently gotten an angora bunny, female, about a year old. She's really sweet, but lately when I have her on my lap after a while she'll start nibbling at my sleeves or sometimes fingers, and digging at my lap. It's never hard enough to actually hurt, but she gets very determined. I've heard that sometimes rabbits can do this as a sign of affection, like to groom the human back, but where's the difference between friendly nibble and aggressive biting?
She likes digging a lot in her cardboard box, but she hasn't showed much interest in chewing any of her not-human toys.