Guys! why would you act like a stranger with your girl best friend in group? :/?

[Hey You!]

we had history before but now just best friends.he is very outgoing talkative guy,he talks to ALL girls and has lot of female friends but he only talks to me when my and his friends aren't WITH us,through facebook and texts.though he had talked where our friends can see us talking privately from afar but not when in group/with us.

i asked him sometimes if he is embarrassed he denied and apologized.again when met,he kept talking to my friend,but didn't even look at me or talk to me like i'm invisible.i was right in front of him.he always does that specially with my friend.even when alone he never looks at me in eyes while talking to me but used to stare from afar when we had class but class is over now.he does tries to impress me and gets nervous.(he told me).

is he embarrassed of me?ignoring me to make me jealous?Many people (even strangers) and he also had called me beautiful before many times,so according to them i'm not that hideous. :/