I have thought about this, and it can be due to any combination of the following 4 things:
1) An unfortunate turn of the neck during a rather enthusiastic sex act.
2) Poor sleeping habits as of late lead to short amounts of deep sleep allowing for horrible positions of the neck.
3) Sandal season requiring quick turns of the neck to see everything and everyone.
or, the most accurate, unfortunately:
4) High levels of stress coming from family, SM work, and the real life job.