S.R.
I would say she is doing it out of excitment. 8 months old is far too young for her to comprehend "hitting" and "hurting".
She probably likes the sound, and 'O' shape your mouth makes when you say "no".
Maybe you can try to condition her by putting her own every time she its you for a minute, also you can do other touching when you are face to face and get her used to that.