I noticed a bump on the outside of my upper left arm. it's about the size of a quarter. it's where fatty tissue is located on ones arm.?

Cyst or lipoma?: You didn't mention any pain so it's like likely to be an infectious process like cellulitis or abscess. Cysts & glands can often swell up as the pore gets clogged then resolve on their own w/o getting infected or causing pain. Lipoma is a benign (non-cancerous) fatty growth just under the skin. It rarely changes size. Best to go see your family doc for an exam & diagnosis.