Large egg sized lump on inner forearm. Just realized it was there when on the computer. No bug bite, not really painful, what could it be - cause?

Lipoma or Cyst. A lipoma is a growth of fat cells in a thin, fibrous capsule usually found just below the skin. Lipomas are found most often on the torso, neck, upper thighs, upper arms, and armpits, but they can occur almost anywhere in the body. Or an epidermoid cyst is a closed sac under the skin filled with a cheese-like or oily material. Symptoms: the main symptom is usually a small, non-painful lump.