Pain similar to that of a bruise on back of upper arm for months now. no bruise or injury visible, jut slight lump, barely visible. what could it be?

Needs to be examined: Any lump or suspected lump need to be checked by a physician. So go pay a visit and find out what it might be. He/she may do an imaging study like an Ultrasound examination to verify its nature.

Torn tendon: Bruise of posterior arm could initiate residual hematoma with resulting resolution of intital skin traumatic changes. Question as to rotator cuff or other damage depends on ability to raise arm above horizontal level. Lump could be fibrosis of hematoma or end of torn tendon with fibrosis. MRI of shoulder might be helpful. If not orthopedist should look at nodule with regard to possible bx