This baffles me

[dc]

For some reason, I have a bruise — a small one — between my left hand’s third and fourth metacarpophalangeal joints, on the palmar side. This appeared in the middle of yesterday afternoon, sometime after 15:00. I have absolutely no idea where it came from In the first place, I don’t recall bashing my hand off anything (and it’s pretty unusual to do that with the palmar surface anyway), and during that period I was sitting at the computer. I was using the mouse, but I don’t rest my hand on the mouse, certainly not that hard. It’s not hugely painful, though it is interfering a bit with typing and mouse usage. I’m just puzzled about how I got it. A bruise appearing in the morning because you have done something in your sleep — that happens; but a bruise appearing while you are sitting quietly at the keyboard — how does that happen?