Hello from JustAnswer.
This is most likely a collection of blood in the tissue, which would also be called a hematoma. With many blunt injuries, blood will leak out of small blood vessels. In most cases, the blood is diffusely spread in the tissue, and if involving the skin, it is visible and is called a bruise. In some cases, though, the blood that leaks out of the small blood vessels will form collection of blood in a lump. The blood will then clot in the tissue. It is this clotted collection of blood that is called a hematoma.
It is important to note that while this is a blood clot, it is vastly different than a blood clot that forms within a vein. A blood clot in a vein is the blood clot that may break off and travel to the lungs. However, a blood clot in the tissue carries no risk of breaking off and traveling to the lungs.
Like a bruise that occurs from blunt trauma, a hematoma will heal by being gradually absorbed by the body. If there is any soreness or pain, then applying moist heat can help ease the symptoms. If it is very large, then it may warrant drainage, but the bast majority do not need any treatment.
If I can provide any additional information, please let me know.