What does it mean when a guy won't talk to you in public?

There's this guy who I have known for a long time but only started talking to recently. It's all been over the internet or via text that we've been talking, and a little bit in person when no one else is around. But what I don't get, especially because this happens every time I get close to a guy is that when we're with a group in public he won't talk to me. If I talk to him he'll talk to me a little bit but I don't want to seem too clingy. Especially because I almost feel like he's embarrassed to be seen talking to me in public or something. And then sometimes when I get home I have a text from him and we start talking like nothing happened. Sometimes I wonder if it's really him that I'm talking to.
What does it mean when a guy won't talk to you in public?
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