Friend trouble?

[Lisa]

I really need some people's advice on my problem.

So, my friend Rebecca and I have been best friends for about 5 years. We were a lot alike, and we basically never got in fights. In high school, we sort of became a "threesome" with this other girl, Katherine. Together, we were really good friends. Well, lately Rebecca has been really moody. Sometimes she is in a really good mood and others not . Then randomly, she decides she is mad at Katherine and I because we did not take her seriously. Well, we apologized even though we were really confused and she finally forgave us. But she is still really moody and now she does not even have lunch with us anymore. She hangs out with a total other group of people. I am really sad, and I do not know what to do. I don't want to lose my best friend.

[???]

talk to her. or just shun her.