Dude it's an infection. It should go away in a week or so. Be carful and watch it it could turn into a infectious hematoma. I had one in my right tricep. Went to the emergency room after about a week of this infection that became larger and very painful. I actually became very ill from the infection. Admitted my self to the ER. Turned out I had a huge hematoma staff infection. They called a emergency surgery to remouve it. It was the size of a grapefruit and my blood was all infected making me very ill. They took it very serious and told me I got to the ER just in time or it could of been very bad( fatal). Not to scar you dude, I have had many infections before that resolved them selfs.