L.S.
My best advice for this is to talk. Talk, talk, talk. Talk to her about everything. If this is a new thing, then something is definately going on inside her little head that you need to hear. Even if she's apprehensive about it, get it out. I was always the kid that told mom that nothing was wrong, and she left it alone. Was not a good idea for her. I drive my eleven year old crazy, but he will come to me with anything now. I've always told him that if I don't know what's going on then I can't help the problem. Fifth grade was the hardest. Kids are changing, they get into their little cliques, and shut each other out. It could be something simple, but then again, it may not. Either way, if you can get it out of her, it will make you both feel better.
To meet new people: Get her involved in an activity outside of the house. Dance, or swimming, soccer......There's a ton. Even a youth group. This gives her a chance to meet people outside of school. And the activities give them a renewed sense of confidence.
Good Luck. :)