What Causes Cracking Sound In Neck When Moving Head?

I sprained my neck about a month ago and I've been going to my doctor and physical therapist to get it better. My neck and back crack all the time though-when i take a deep breath, moving my head, even when I'm lying down. Why is it cracking? And what is it actually doing? Is it bad for my neck injury - cause sometimes the cracking really hurts but mostly its just a noise.