How do i know when my guinea pig is happy?

[Jemima]

I am a new Guinea Pig owner and I'm not quite sure how to tell whether she is happy, scared, angry etc.
Whenever I put her on my bed to play with her she runs around squealing quite loudly, jumps around and then looks out the window squealing some more. Or when she is in her cage she will just out of nowhere start squealing. I am quite concerned because I really don't want her to be scared etc. I know that she is a social creature and I am getting her a companion in the next week or two. Any advice would be greatly appreciated! Thanks!