Sorry if this offends you but that's kinda funny. She was only mucking around with you for fun. In return why not give her one back?
That seems so ...weird. How old is she? 12? And you ignored it? I think I'd ask about it. Do you guys joke around like that or was this from left field? If she's your age, I think I'd ask what's up with that. It's not how I'd spice things up.