Oh, how horrible for you.....I'm sorry
How do you think it happened - maybe you were sleeping with a window open and a draught caused it?
I hope you get relief from something - I remember it happening to my sister, and she was told to apply freezing cold, then warm.
If you do use frozen peas or an ice pack, make sure you wrap it in something first, so it doesn't cause those nasty 'burn' feelings....
Hope you feel a lot better soon
Sally x