Some guys form cysts from thing like pellets. I have one in my right bicep from getting stitched up (29 stitches) after a fall, the idiots at the hospital didn’t put in a drain and I formed a fluid pocket in there that never healed right, even though I realized what was going on and started draining it myself a couple times a day until it healed.
Location is also suspect, I always alternate the upper outside quadrant, whereas this is lower left where I would never be able to reach.