My friend changes around other friend. Please give advice.?

So I have this friend and her other friend was gone away on holidays for two weeks and my friend and I got closer to her ( we were already pretty close) we got on great and went loads of places together. Her friend came back this week and she's been acting differently and I don't like it. Nothing major, she still friends with us and whatnot but its just like she's acting different, saying things she wouldn't see before. I'm left confused whether she's changing for her or if she was acting differently the whole time she was with us. I feel really sad for some reason and I feel like I lost her. Today I was talking to her and her friend wasn't there but she was less different, but not completely. I don't know what to do because I really love her as my friend. Could ye tell me why she might be doing this and if so she i say something to her. Or just let it go ( which I would find really hard) please help!