My bunny keeps nipping me??

Question

i have a six month old giant chinchilla bunny called magilla, he's not desexed but i don't intend to breed him. he's litter trained and very friendly (he runs up to me when he see's me coming and stands up so i can pat him) but sometimes when i'm sitting on the ground he'll run up to me and nip me then just stay there. he doesnt look scared or angry and he doesnt run away or anything. does anyone know why he does it or how i can stop it? sometimes it really hurts!!!