What could a sudden painless large lump under the skin on your upper back by spine be?

There are several possible causes for a sudden painless large lump under the skin on your upper back by spine. Some possibilities include:

Lipoma: Lipomas are benign fatty tumors that can grow under the skin. They are usually soft, moveable, and painless.

Fibroma: Fibromas are benign growths of fibrous tissue. They can be firm, smooth, and painless.

Hematomas: Hematomas are collections of blood outside of a blood vessel. They can occur after an injury or trauma. Hematomas are usually tender, swollen, and discolored.

Abscesses: Abscesses are collections of pus that can form due to a bacterial infection. They are usually red, swollen, tender, and warm to the touch.

Cystic acne: Cystic acne is a type of acne that can cause large, deep, painful cysts to form under the skin.

Swollen lymph node: A swollen lymph node can occur due to an infection or other medical condition. Swollen lymph nodes are usually firm, smooth, and painless.

It is important to see a doctor to determine the exact cause of the lump and receive proper treatment.