Not unusual at all. I am presuming that your surgery involved a neck dissection, which inevitably leads to some degree of lymphedema. This is the lymphatic fluid that used to be handled by all of those lymph nodes they removed. It is looking for a new way out, so pefuses the surrounding tissues. Sleeping position, weather and a variety of other things can cause the swelling to increase or decrease. I am about a year out from the end of treatment, and mine is better, though not totally eliminated yet.

Hope this helps.