I'm 9 mos out and have been experiencing exactly what you've described. Sometimes the smallest, least strenuous activity, like walking with my laptop bag over my shoulder, will cause a cramp and send a sharp pain down the side of my neck. Fortunately it goes away after 5-10 secs. This happens about 5-6 times per week. I mentioned this to my oncologist at my last check-up, and he attributed it to radiation damage and did not offer anything to help.