I slept funny and ended up needing physical therapy.
Woke up one morning on my stomach, with my head turned to one side. When I got up, I realized it hurt a lot when I tried to turn my head to the left even 1 degree past straight ahead. I’ve slept funny before, and it usually went away after a week or so.
Four months later, I was still living with the pain and the limited range of motion. I finally got around to visiting my doctor and he referred me to a physical therapist. Turns out that my neck muscles were overcompensating to support my head after I slept funny, and this caused my lower back muscles to weaken. Since my lower back muscles became too weak to help support my head, my neck muscles had to keep overcompensating and the pain persisted. After a couple months of stretches and lower back exercises, my neck finally returned to normal.