Why is my guy friend acting off with me?

I’d consider us friends and we were maybe not the closest of friends but we would hang out and chat and he even made out whole class sing happy birthday to me and has added me to his private story. But recently the past few weeks he’s been kinda off. Like won’t say hi or bye to me when we are hanging out in a group and he is an outgoing person who talks to everyone but goes quiet when I join the conversation. It makes me kinda sad tbh cause I honestly thought we were friends. Do you think I’ve done something? I can’t seem to think of what could’ve happened that made him act like this around me. I guess I wanna know if this is a sign that he could be mad at me or just finds me annoying
Why is my guy friend acting off with me?
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