I have a knot under my left arm started out the size of a quarter now its the size of a baseball could it be a fatty tumor and is that cancer?

Could be lipoma: Lipoma are not cancer, but depends on how fast it's growing. If it's within few days I would be concerned about infection. Of months to years a lipoma is likely, but it's not a bad idea to have your primary doc take a look

See your doctor.: A lump under your arm can be an abscess (infection) or a tumor or a cyst. You should be seen by a doctor to make the proper diagnosis.

See doctor: to determine what the mass is. Lipomas are not cancerous. But we don't know what the mass is. Diagnosis then treatment.

See a DR: Need exam to decide if cyst or something else. Whatever it is, it is so large you will want it removed. Then it will be possible to define it. Treatment follows according to the nature of the mass.

Axillary mass: A mass that grows quickly from the size of a quarter to the size of a baseball could be a fatty tumor, but may be an infected cyst or lymph node. These masses are usually very painful, warm to the touch, and rarely cancerous. Cancerous lymph nodes are often not painful. Seek medical attention ASAP as this type of infection can spread to your blood stream.