Biting and pinching and scratching, oh my???

ruby is almost 13 months. she is very affectionate, kissing and smiling all day long. the last two days, though, she has started biting or scratching me when she is tired or wound up. when i say "no," she laughs!!!! (which makes it hard for me not to laugh, which i know will reinforce the behavior. anyway...) but, really, it hurts! she has 4 teeth, and those little fingernails are like razors. i am guessing this is normal? but i am a bit confused as i have never modeled ANY physical aggression in front of her. so where does this come from? how do they learn to hit/bite/scratch? and is it actually normal? i am not super worried about it -- just looking for some explanations, insight, advice, whatever you've got. thanks.