2 Days Ago, I Discovered A Quarter-sized Lump On The

2 days ago, I discovered a quarter-sized lump on the inside of my buttock. (It is under the skin surface, and not in the rectum area) It isn't painful, and moves around when I feel it. For the past six months, I've worked at home, and spend 10 - 12 hours a day sitting at my computer desk. I'm wondering if the excessive amount of time sitting may be a factor.