Why is he acting this way if he has a girlfriend??

One of my closest guy friends has been acting a little differently and im not sure how I feel about him. All of a sudden I find myself wanting to talk to him whether it be online or in class all the time and I smile whenever I see him walk in the door because I know he’s gonna make me laugh throughout the class and brighten my day. He is a flirt, but now its gotten a little more than just “friendly flirting” you could say. I’ve known him since 7th grade and we’ve been in at least one class together every since freshman year of HS (were seniors now). He always makes fun of me and jokes around with me, some people may take that as flirting. last year, even our teacher asked our other friend if we were going out!! (we werent and never have) he sits right behind me in class this year and will sometimes play with my hair, we both play volleyball and he wants me to go to every game. I missed one home game last week and I had to make him brownies so he would forgive me and he now expects me at every home game. We are always partners for projects. Last year, us and our other friend had to make a music video for a project and we did it to “love story” by taylor swift. I was “juliet” and he was “romeo”. Then this year, he left me a voicemail on my phone at 1am and he was singing the song! He had no idea what the lyrics were but he was saying “I am romeo and you are juliet” over and over and “its a love story baby just say yes”. when it came up a few days ago, at first he claimed it was his friend singing but then later he said it was irrelevent to what we were talking about. The wierdest thing is, HE HAS A GIRLFRIEND! and they’ve been going out for awhile, last year when they started we all thought he had had a girlfriend just so he could have a girlfriend but there still together and on a social site there “engaged”. why would he do all this and act the way he is if he has a girlfriend???

My friend thinks he likes me, she says the way he talks with me and his girlfriend are completely different

what do you think? sorry this is so long but im just confused and not sure if its just innocent friend stuff or something more.