A bit complicated can you help? boys and girls?

Ello,im 17 ok so i met this girl at during the weekend. Bear in mind i kinda used to talk to her on msn around 3-4 years ago not much just briefly. but then just lost contact.anyhow on the weekend at times i got the feeling she was starting at me, and at one point said something to her friend and both looked at me, she came up to me and i then realised her and said hi etc, talked very quick cos my friends were around being quiet annoying. next day start talking to me first on facebook, the general opening hi hi etc. then had to leave to a concert came bk again she spoke to me, there was slight flirting in our convos. But then i saw she was in a relationship with a guy but hes not even in the country. Wot can i do if its serious or not? and does she like me or happy, she doesnt go to my school and i woudnt say im friends friends with her.