I say smoke and don't tell her.
It's none of her business.
It's not hurting her or anyone.
Don't let her find out, until (or if) you are ready to tell her.
You are 17... you are not married, you are not living together. You aren't smoking in her house or around her. You made it sound as if she made you quit... fuck that... her trying to make you quit is WAY more wrong then you hiding it from her. Some people will only look down on people that smoke... that's their loss, it doesn't have to be yours. If you can't change their mind then just become a stealthier smoker.