It could be anything. A food, a drink, a habit, etc.
For me, I sleep on my stomach. Horrible for the back and neck, might even be why I get tension headaches occasionally, but I just can't fall asleep any other way.
Is that too bad? :o I never sleep not on my stomach! It's the only way to sleep for me. :(
Well, I haven't had any problems from it yet, but it apparently screws with your spine and neck muscles.