What Causes Knot On Right Side Of Neck?

I am 5'2'' and 159lbs and I have a knot on the right side of my neck where my pulse is. It is the size of a nickle and hurts to the touch. I have an appointment with the doctor tomorrow but I would like to know what it could possibly be. I am very concern. It is above my thyriod and I do take 50ml of synthriod.