Can you help me with a friend?

There’s a girl at my dance studio (and school too) I’m getting worried about. She’s been moody in a quiet restrained way for a couple years, that if you’re not looking for it you can miss it really easy. She’s also been really quiet.
This summer she was supposed to go to a dance camp but I happen to know that she didn’t really, and took the summer “off” and stayed at home. She came back and had gotten way out of shape, and noticeably larger (she was only out for a little over a month…)
She loves dance, but she hasn’t been coming to classes, calling in sick. But when I see her she doesn’t seem sick…? She’s also not been coming school—again, sick.
When she is at either, while she’s talking more she seems worse to me somehow.
She’s not turning in assignments at school, and she’s dancing with very little energy.
She’s very closed off so it’s really hard to talk to her… so I thought I’d see if any of you have any idea? I know I haven’t given you so much to go on but I thought I’d try?