Why did my crush smile at me, he would not pay attention before

So there’s this guy I have a crush on. I switched maths class in September so I was like the new girl, I wasn’t introduced or anything I just sat down and no one cared. In the second week, this boy turns around and looks at me and then at the end of lesson turns around and looks at me again. I was confused because no one turned around and looked at me as to who I was, but I was like maybe it’s because he didn’t come in the first week so he probably just realised now that there’s a new girl.

The next day for my other maths lesson, we have two so for this one the seating plan is me right at the back and him right in front of me. Me and the guy next to me would speak and my crush just butted into the conversation and tried to speak to me. He did this again the next week. Then whenever the guy next to me would ask the teacher for doubt my crush would use it as an excuse to turn around and look at me.

Sometimes he turns around and looks at me at random times but that’s only a few times most of the time he does it because the guy next to me asks teacher to come help him so uses that as an excuse to look at me. He only started doing this after I walked past him and gave him a death stare this one time,( which was like a few days after he tried talking to me twice) after this one time he started doing this btw. One time it was home time and he didn’t really do anything. But before he was about to go to his usual way home he stopped walking and turned around and started looking for me. Once he saw me he turned around and continued walking

Now for 1-2 months he ignored me he never looks at me never really paid attention to me. So I stopped liking him and now today we had a maths exam in a huge hall and I walk out from where I put my stuff and he comes from the other side with a smile on his face looking at me, he wasn’t smiling at me it was more like he smiled to himself when he saw me.

I don’t understand why because he ignored me for like 2 whole months even though before he used to turn around and look at me in class at random times