My 4 Year Old Son Has Fell While Outside Playing.

My 4 year old son has fell while outside playing. He is watching T.V and nothing has changed in his behavior. Yet the bump seem like its full of fluid. When he first hit it he came it was the same way. Got it to go down but it has came back to surface. He says it hurts. He has had stechs before. He was running at church and fell into a brick step.