What Causes Pain And Cracking Of Neck?

20 year old male, neck popped loudly (a few months ago) upon waking and stretching, hurt upon moving a certain way and healed in a couple days. A few days ago he was having a bad dream where his head and ears hurt and was about to explode. Woke up and heard a loud sound and couldn't move. I think his neck popped again.Any thoughts?