I am generally truthful person because lying just doesn't feel right to me. So why is it that whenever I am telling the truth, I feel as though I'm lying, and I accidentally give body language that I am? This is very frustrating, how to stop it?

It's called lack of self confidence or that you don't truly believe in yourself and due to that lack, you feel that you're portraying yourself in a way that no one believes you even though you're telling the truth and if someone questions you, you fall apart and turn shy. In time you will build up your self confidence and never worry about this again.