What do you do when you get nervous?

I either ramble, stay quiet, look around, no eye contact, heart races, blush if I'm called out or something I like happens, etc...

Also; I got nervous around this girl I'm really close friends with who honestly saved my life.
I was nervous when we were alone for the first time but I'm comfortable around her whenever were with people. We've known each other since the start of junior year and were now seniors.
Why do you think I'm nervous when alone with her but comfortable around her when we're with other people?