Friend...ish?

Hello guys, I am 17 years old and I am a female. I'll give you a brief introduction on who I am. I'm basically what you can call a tomboy. I wear oversized clothes, love sports, and I hang around with only guys. I've been with my current friend group for 5-6 years, and I realized something that had been weird.

There's this one guy that I found had been acting quite....weird. Recently, he had been doing stuff like ruffling my hair, squeezing my cheeks, and laying on my lap. He had been taking pictures of me as a joke and I did it back and eventualyl we would realize we had enough to threaten each other. Considering I had never been in a relationship, I found this unusual, but I realized I had been starting to be attracted to him. I'd say he's one of my closer friends, and we play games together and talk often. These actions had been repeating a couple of times, and I'm sad to say that I think I've fallen for him.

I had asked my friend about this, and told her the whole situation, and I was shocked with the response that he had a GIRLFRIEND. She was in another school which had made sense on why I didn't know her. Apparantly they had been dating for a couple of months now, but I was slightly disappointed I had to find out this way. I don't know if his actions were unusual to others or just me. I can't tell if he's treating me like a younger sister, but in other peoples' thoughts, it sounded like he was attracted to me before I told them he had a girlfriend.

His girlfriend had apparantly been aware of me, and she texted me about how I should stay away from him. I followed her on social media, and they are really close and the girlfriend posts pictures of him a lot. I had done nothing wrong or deserved to be in this mess. My mind is messed up right now and I don't know how to handle it. What are your guy's opinions on his actions? How should I handle this?