Why do I do this?

Are you unconsciously doing any Kegel exercises or Kegel crunches? Don't know what a Kegel is? Here is a link for Kegel Exercise.

If not, then follow the other's advice and relax.

It could just be the way your body reacts, it doesn't sound like a bad thing, does it happen when you masturbate?

I've experienced that too. You are nervous. You need to find ways to make yourself feel more relaxed.

It probably means that you are nervous rather than fully into it..

It could just be insecurities. You might be worried that they'll think you're not good in bed, especially if you haven't had many "experiences". You might be worried about smells down there. You might be worried that it will hurt (especially if you had a painful first time).

It could also be part of your upbringing (disclaimer: I am not intending on bashing your parents or religion). For example, I was brought up in a Roman Catholic household where sex for pleasure was considered dirty and immoral in an attempt to keep me as "pure" as possible for as long as possible. Maybe you had a similar upbringing?

If you get close with a guy, talk to him about your feelings- he could be feeling the same thing. And getting that stuff out in the open will make you more comfortable in bed. Good luck! :)