Many things it could be, it actually needs to be assessed by a medical Docter, we cannot tell you what it is or isn't from a description, it could be many things that are harmless, or something sinsister, do you have any health problems aside from this lump? I recommend seeing a doctor if you have insurance or a free clinic if not, trust me dude the last doctor I seen was in prison and that was 3 years ago, they work for the moment, go see a free clinic if it worries you that much.
B