joy vs. public speaking

i don't know what's wrong with me, but whenever people stop talking to listen to what i say, i FREEZE. in a group, i'm fine. like singing on stage, in a group, fine. that's how i did that a cappella thing. but standing in front of 300 people, no. sitting in a circle of 5 even, no. i'm horrible. what the hell happened to me when i was little that i'm unable to speak in public? :(