my 6 yr old son was running on the playground today and hit the inside of his upper arm on a metal bar. he complained about it earlier and when i touched it, he winced in pain. i thought it would go away, just like a normal minor impact injury. there is no bruise , only a small pink spot, but there is a hard gumball sized lump just under his skin. is this something i need to worry about? it is not any warmer to the touch then any other part of his skin and the rest of his arm(and hand ) is perfectly fine in appearance and temp.