Am I embarrassing?

Lately I feel as though the guy I have been seeing may be embarrassed to be seen with me. We have became oddly close ever since attending a mutual friends party, however his advancements only come when we are alone or with a select few close friends. He seems comfortable with talking about us as a couple as well as kissing me in front of our best friends. Out to lunch one day we were holding hands, his friends arrived and he immediately pulled away, pretending it never happened. At school he doesn't come as close or touch me unless we are alone or its out of view from other people (under the desk). I think he may be embarrassed to be seen with me and I don't know how to ask him about it.