why is my best friend becoming distant?

I have noticed that my best friend (lets call her sara) has left me out!? A couple days ago I was with a group of friends (sara, 3 other friends, and I) and we were walking in the hallway to class. Two of my friends (lets call them holly and thym) come up to me and start talking to me. Behind us is sara and another of our friends (lets call her jadi) are talking. They catch up to holly, thym, and I and all of sudden sara starts pushing me behind them. Then I end up walking alone while all four of them are walking in a group. Today, sara came up to me at our lunch table where we usually sit (just sara and I) and she looks back where our other friends sit. She told me she wanted to sit there and she then got up and said "I'm sitting over there". There was only one seat left since all of the other three sit there. So I was left alone at the table but then one girl came to my table and said that she saw me here alone so she sat with me. I ended up talking to her and she invited me to her table right behind the table sara was sitting at. Since I was so angry at sara I walked right past that table without looking at her and I sat at an angle where I could see sara out of the corner of my eye. Almost every 5 minutes I saw her look at me and the next class I ignored her and so did she. I talked to the other girl that invited me at her table and sara hung out with the others.

I feel so left out and confused. Why did she leave me? Is she trying to tell me that our friendship is over?