Biting

Our little Margo is an 9 week old JRT female. She has always bit our hands, in a playul manner, but this last week she has been viciously biting on occasion. I have four good cuts on my hand right now. We play with her for an hour or so each morning and take for walks at night, as well as plying fetch with a tennis ball and playing with her toys. I think we give her enough exercise in a day. Is this type of biting normal, and what can we do to curb it. We have tried time outs, ignoring her and yipping when she does this.