I have a 4-5cm lump on upper arm / bicep, is it really a cyst?

For a few years I have a 4-5cm lump on upper arm / bicep. Firm but movable when manipulated. Sometimes gives mild twinge of pain. Slightly raised if skin is held taut. Just under surface of skin. A doctor once said it’s a cyst, don’t bother.

Age: 59
Male
Medications: NSAID