I rarely get bumped enough to bruise or swell my middle finger but it happened lately. I had rotator cuff surgery on my right shoulder this winter. Tt has significantly slowed my gun mounting (I shoot trap from low-gun), and I think my elbow is even lower than normal. These two things contribute to a slightly different hand position and results in a bump. Once I start forcing myself to pick up speed and lift it a bit higher, this all goes away.
In any event, if anything is affecting your shoulder, wrist, elbow, or whatever on your trigger hand side, I would not be surprised if you started getting bumped like that.