There isn't sufficient information to suggest a specific diagnosis. It is possible that it could be a muscle spasm due to improper posture while sleeping or otherwise. I would advise you to put a hot water bag over the area and massage the area gently with a topical pain killer. Kindly also aim to drink sufficient water (2.5-3 lietrs of water/day). If it doesn't resolve kindly visit the ER so that you can be examined and undergo necessary investigations to find the underlying cause and have it treated accordingly.