I Have Constant Pain On The Back, Left Side Of

I have constant pain on the back, left side of head that stretches to the base of the neck and even the collarbone. It's not sharp pain, more persistent and feels more intense at the end of the day. It feels like a big knot from the back of the head into the base of the neck/shoulder. Last night, my collarbone area even felt affected. What could it be?