Bump On Head, Been Growing, Painful When Touched. Worry?

My four yr old son fell off a couch and bumped his head on a wood cocktail table. He cried but not abnormally long or anything. He only let me feel it for a second, but there was not an immediate bump. The following evening while givin him his bath, he too me to be careful washing his hair because of his owie that still hurt. There was a bump, but it was not raised much. It has now been 3 days and tonight when I rubbed the back of his head I felt a bump the size of half a golf ball. It s about halfway between his the back of his ear and the back center of his head. His behavior has been normal. It hurts when its touched, according to him, more than when it s not being touched .