Lump back of head left side lining with bottom ear about 2 inches away?

hello, i was wondering if anyone could tell me what this bump is on the left side on the back of my head lining with the bottom of my ear just 2 inches over. it is on one of the two bones you can naturally feel that's on both side. It's a hard lump about the size of a nickel maybe bigger, it doesn't hurt or anything. just wondering if anyone can tell me some answers and bring some peace to my mind thanks.