Do I have social anxiety?

I'm usually a very social person when I'm with a few people, but when a group I'm having a conversation in exceeds four people I seem to have a problem talking. I just stand there awkwardly and listen to the conversation, laugh at jokes and such. But I have trouble actually contributing to the conversation. All of my thoughts leave me and I literally stand there speechless. So do I have social anxiety? If not, any idea what?