Fountain of Youth
The only time I had such a horrible stiff neck was about a week before I had a full onset of Mono confirmed by a visit to the doctor. Funny thing is that it happened one Friday night at my boyfriend's house, he was tickling me and then *boom* I couldn't turn my neck to the left. It was not easy driving home, as I had to merge from an entrance ramp into suburbian Detroit traffic. I had to turn my whole body to do that. By the next day at work, I was still suffering.
My boss is an old hippy, and one of his friends from the '70's just happened by. The friend was a naturopath. He offered to work on my neck, and I took him up on it. While he was giving my neck a lymph massage, he said that it felt like I had a kidney infection because of how my neck felt at a certain spot. He suggested drinking cornsilk or red-root tea to help clear out the kidney infection. Needless to say, I didn't and by the next Thursday, I was severely tired and my tonsils were so swollen that I could barely swallow.
About a week after this, my sister was having similar problems and started blaming me because she thought she might have mono. She went to the doctor, was tested, and found out that instead of mono, she had viral meningitis. Our symptoms were very much the same. Of course, if your pain goes away none of my story matters, but if you start running a fever or having a sore throat, or other problems, you might consider my story. I certainly hope that it is just the usual -> sleeping on it wrong. Best wishes.