Our Boston has bit me twice. Please Help.

My girlfriend and I got Leela about 3 months ago from a rescue shelter. She has been the sweetest do to us. She loves fetch, she never barks, and she just loves being around us. She has always been a little wary of other dogs due to past abuse, but she seems to get along alright with most, non-hyper dogs. I was sitting on the couch with Leela and my girlfriend last week. Leela started to sniff me very quickly sticking her nose into my chest. I put my nose on the back of her neck and mimicked the sniffing sounds. She spun around and bit my face hard enough to draw blood. I punished her immediately after with a stiff smack on the ens of the nose and a time out in her chair.

Last night, approximately a week from the last incident, I was putting Leela to bed beside my bed. I rubbed her and scratched her like I've done dozens of times before, but when I got up to get into bed, she bit my thumb. She hasn't growled before either of these incidents. She definitely sees me as the alpha male and my girlfriend never punishes her when she does something bad.

Any insight?