I have a friend issue.. advice please?

2 months ago I invited a friend if mine to eat at a japanese steakhouse for dinner on my birthday. We had a great time and everything. I've been friends with her for the beginning of the school year and now everytime I ask her if we can hang out, its always "Oh i'm busy babysitting." " I'm grounded." Then on facebook I see that she always put statuses about hanging out with her other friend at the mall etc on the day she told me those exuses. Then I think, well aren't you grounded and aren't you babysitting? I'm not jealous but sometimes I think she is lying. In school she talks about having a pool party in the Summer. She talked about everyone she was invtiting but she never asked me. The reason why I know for sure is because right in front of me, she said to someone else " Yeah, I'm done inviting people." I thought.. well I invited her to dinner and she never wants to hang out. Can I have advice on how to talk to her about it. Maybe she's not a true friend and she could be using me. -__- Oh and she talks to me in school and acts nice.. so thats whats really bothering me too.