How do I get this girl off my back?

[Nicholas]

Basically my youth group met up with another youth group for a weekend and this girl was there and she was acting kinda weird, I spoke to her twice (really short) and almost every time she saw me she said hi. Our youth groups have met up a few times since then and she keeps doing it. And then yesterday she added me on Facebook and I accepted to not be rude or anything as I had added others from her youth group, but almost instantly she starts sending me messages and I'm like, hi, yeah. And she was like "looking forward to seeing you next time" and it's kinda creepy and then she invited me to her baptism and all I know about her is her name and what town she lives in and I didn't reply to the messages incsse it gave her any ideas. And now she is liking photos I'm tagged in and stuff and I don't know how to get her off my back without being rude or making her upset. Any ideas?