My cat is acting wierd...?

So I have a cat. She is pretty old. (indoor cat) She must be atleast 15. And lately she hasnt been eating when we feed the rest of them (we have six). She is usually out there eating with them. But now she just sits in the bedroom. She doesnt seem sick, Cause she is still active or so. She will go and eat some dry food though. I just am worried about her. Whats wrong?