My friends are acting weird?

Today my school year went on a trip and on the coach I sat next to my friend (let's call her Anne) and my other 2 friends (let's call them Millie and Ellie) walked past us and sat far away from us. They didn't talk to us for the whole trip. But when Ellie went to the bathroom Millie started acting really friendly towards me but she was acting like Anne wasn't even there. But when Ellie came back she ignored us again. At the end of the day Ellie was acting friendly towards me and Anne. Ellie is a nice person but I think she gets sucked in by Millie if you see what I mean. Anyway what shall I do? I think Millie was b**ching about us too to Ellie.