I cant tell you what it is...but I am going through the same exact thing today is day four. I jus been putting a heating pad on it and taking aleve. If you find out what exactly is causing this please share. But I dont believe its from the way i sleep, i slept the same way and on the same pillows i always sleep on. I DO NOT want to go to the hospital, so your help would be appreciated. Mz. Bonnie Divine