It does sound like a muscle spasm. If what you're doing already, combined with stretching and pain-free active range of motion aren't helping, I would suggest consulting with a chiropractor. This is one of the top complaints I hear from patients, usually starting with "I woke up and couldn't turn my head!" Gentle adjusting, massage and stretching instructions could really help, and a consultation to rule out anything more serious would be a good idea. Best wishes for a quick resolution to your pain!