My son has a rock hard bump under the skin of his forearm it is not outside noticable but you can feel its very hard what is it? it is a hard bump under his forearm. he was playing and barley bumped it when he was playing with his lil brother. he is three

The : The most important question here is if the lump was present prior to the incident where he bumped his arm. This is a common, the discovery of a lesion that is thought to be from the injury that occurred but was actually present prior to that. The bottom line here is to have your son evaluated by his primary care provider. It may just be a simple cyst or lipoma, however it needs to be seen. Best of luck.