Hi Chicken Soup,
My sister who suffers for Bipolar and A/P had the same problem. She thought it was a side effect from one of her new meds. Found out after talking with her doctor about it that it was a pinched nerve in her neck. They sent her for a bunch of x-rays and Monday she will be starting physical therapy.
I would say if it persists to get in to see your doctor.
Good Luck and Keep us posted.