Why is my guy friend acting weird?

Hiya, I've been friends with a guy for nearly 2 years.We joked around in lower school and teased eachother (the nice way). I suddenly began having feelings for him at the end of our last year. He was my best friend and we even hanged round with eachother at the end of school.When summer was over we moved to upper school. There were loads of people who were strangers and we decided to stick by eachother. We were tight for the next 3 months and were always lab partners. Then the next month everything became different. We started to drift apart slowly and I noticed slight changes in him. Like he always used to talked to me and at break we used to see eachother. He also started to get popular friends ( most were stuck up) and was acting differently around me. When he looks at me for instance his eyes dart everywhere and he looks to the floor usually. I was lucky to get a sentence of conversation every two weeks. There is this one girl that he is friends with and she flirts with him a lot. Like she ruffles his hair, hugs him at awkward moments and plays footsie with him. He looks really uncomfortable though. For instance, when she hugs him he tries to back away and sometimes looks at me and then somewhere else. I think that she does it to annoy me because she dosen't do it to any other boy and it is always where I can see them. I was in the field one day and I noticed that him and his friends were looking at me, In a weird way. I don't know what was going on but they were smiling. Another strange thing happened. I was walking in the corridor to go to the library and him, his sister and two other boys were standing by the Walls. They also were looking at me weirdly too and they said something but I don't know what they said. A week ago as well he was telling all these jokes and I swear that he looked at me 4 times to see if I was laughing. I don't have feelings for him anymore, but he has changed his hair and wears aftershave too. I am confused, Thx for reading this :D x