Why am I jelous of my best friend?

My best friend is like my sister. But when were out in public guys always want her. They want her at school dances (which is why I dont like going to them) school, and anywhere else we go. I am happy for her but always when she says she's doing something with them I put this face on. I say im mad but I know it's jealousy. I've never had a guy want to talk to me or go up to me and I never had a boy friend. I don't know what wrong with me lately but my best friend knows therea something wring and I don't want to tell her I'm mad with her because I'm jelous. So what do I do?