It's because of your insecurity girl, it's just that you don't feel safe around the people, because you think you mess it up, make a fool, look weird, feel shy, feel anxious, feel ... sooo many things can be that.
and You should not be looking for the answer WHY you get so uncomfortable, you should look at the answer, HOW to get out of your comfort zone, like feeling comfortable around people , wherever you are standing, just feel good and relaxed around your company and just be fine.
You should ask people like, what do you think I should do to be more confident/comfortable around people.
My answer would be,
1.Confident body language, maybe you don't feel that way, you can still act like it. And eventually you will feel it growing like you feel the gutts to feel confident.
2.Know you are worth it, don't hide when you are around people, show your self to the world, know that you can do it, and maybe you should try to prove yourself, (yup yourself) that you can feel good around people, and smile at them, like nothing is going wrong. And that's what I can having trust in your own actions, feelings, thoughts and behaviour. Don't worry girl, you are the person who thinks you are feeling unsafe, while the other people might think you are a nice person or look fine, or whatever they might think, it doesnt even matter, because its about you, not about what other people might think. it's your life, it's your choice, it's your dream, it's your faith, it's not their decision. So just let them think whatever they wanna think, and you are not responsible for their thoughts, and they should not rule you, and maybe its all irrational, so what were we afraid of?, hmmmm.. no. We should just feel alright, and fight! I know this is a hard road, but one day you will step in the boat, of feeling comfortable around people like you never did before. But you only will do that, if you know you will. and believe it.
GOOD LUCK!