That happens to me. I would say it’s the voice in the back of your head being judgmental. Maybe you are listening to your own voice and wondering how it sounds to yourself and other people. For me it’s me wondering what there reaction is going to be before I finish the sentence and I end up stuttering. Or Rather or not they will take me serious. Also when you speak louder you are using more air so you could be running out of breath. When you whisper it’s feels more comfortable and easier to breath.