You should definitely show it a dermatologist or general surgeon. You say those knots can disappear completely? Do they hurt? A few things come in my mind- neurofibromatosis (but then you would have knots elsewhere), some kind of lymphdrainage problem I have never heard before, lymphnode enlargement due to infection or tumor (but then they wouldnt disappear again), fibroma or lipoma (benign, not dangerous tumor of fatty or connective tissue, which is located deeper between muscles and prolapses through muscle fibers. When muscle contracts, it causes pain, when you push it deeper back in the pain goes away and the lump disappears. When you sleep, muscles relax and the tissue slide between muscle fibers out again.)
If its lipoma or fibroma, it should be surgically removed since it disturbs you. Dont wait too long or it will slowly get bigger and bigger and disturb you even more.