I think she's assuming that walking to a store might be out of the question, and referring to the ability to drive to the store to go and buy tampons without her mom breathing down her neck about not using tampons. I know if I asked my mom for a ride to the store when I was 13, she'd ask me why and go in with me-- my mom was OK with me using tampons at 13, but she'd criticise my brand choices until the cows came home.