Why is he acting really strange around me again?

Hi, so i was friends with this boy at one point when i was in lower school. he came all the way from somewhere in london ( i'm in the UK) and came to my school and my class in our last year of that school. he started to settle into our school and shortly after we became friends. we became quite close and were very much alike as one another. he became friends with my other friends and we hanged out together whenever we had the chance. he wasn't like any other of my guy friends and was a sweet, honest person overall. he was never mean to anyone and even talked to this boy who most people bullied everyday. he had a funny accent and we used to tease him about it and do impressions of him and all that sort of stuff. i was really upset of my best ever guy friend moving to essex after 6 years of friendship, and he cheered me up whenever he saw me. he was quite shy and i was too in fact, that was what made us so alike. after a couple of months we were stronger than ever and i started to have feelings for him. when we finished our years at lower school we didn't talk to each other in the summer holidays. i didn't have his phone number or anything because he was always in rugby practise and was never inside of the house at all. i was really exited to start high school and still had feelings for him, and missed him very much. when we started high school, everything was practically the same and we still were best friends. we were always lab partners and joked around a lot in our classes together. after a while though things were very different. we started to grow apart from each other and made new friends and all that. i never wanted to grow apart, but it just happened for some reason. after a couple of months my feelings for him faded away but i was still depressed though. we hardly talked to one another and we were lucky enough to get a sentence a week of conversation. he started acting weird towards me all of a sudden. when i looked at him in class his eyes darted back and forth or he would just stare after i was finished looking at him. when i tried to make small talk with him he would just look at the floor and then glance up for some reason. he made friends with the popular people too ( most of them were quite mean.) he made friends with this one particular girl who is acting strange too. she flirts with him a lot like she hugs him and ruffles his hair and makes physical contact. i am not jealous or anything, its just that she does it whenever i am nearby. its like she's trying to make me jealous or something, but i don't know why. the weird thing is though that he always looks really uncomfortable when she does it. i know i probably sound like a stalker but i cant help it because she does it right in front of me. when she hugs him, he back away before she can touch him and look at me quickly for some other reason. he always looks angry when she messes his hair up too, i know i should act jealous but i giggle because it looks like she's irritating him. a couple of months ago i was in the field with my friends and my best mate said that there was people staring at us. i looked and it was the populars and my ex best guy friend was looking back and forth at me. his friends were smiling at me and then looking at him. another time was when i was in the hallway and his sister was standing by him and i walked past them and around the corner and she was looking at me strangely. i swear that they said something when i walked past but i don't know what. i was really confused and asked his friends but they said they didn't know either. after a while it stopped and he didn't act awkward around me anymore. but just a couple of weeks ago it started to happen again. i have been off school for a little while because of medical problems and stuff. he does the eyes darting thing again and when he tells jokes to all the class he looks at me about 4 times but i still don't know why. he is acting really weird again and the girl he's friends with still flirts with him and he still looks awkward. i don't know why this is and i am getting annoyed because i don't know whats going on. i haven't done anything wrong and i haven't said anything about him. i am so confused, can you please help me? thank you for reading this, much love xx