My husband has a lump under his skin on his forearm. It is about the size of a penny in diameter. It appeared quite suddenly. It doesn't hurt but it does seem to fill and drain. If he lifts his arm up it seems to vanish only to reappear when he lowers his arm. It is sometimes hard and he will push it until it pops and is soft again. Yesterday when he pushed on it it felt like it was filled with a crunchy substance. What could this be and what kind of doctor should he see to take care of it?