I'm not entirely sure what caused it but it seems harmless.  If my internet research is correct it's a harmless underskin bubble of tendon liquid that sometimes forms when an injury occurs and the liquid quantity increases to fix things.

I'm not sure what injury caused this as I really haven't hurt my wrist recently, but I have been pushing my arms muscles to build up for steel combat.  Maybe it's the combined stresses on the tendons, joints, and muscles.

Anywho, pictures of the weird bump that showed up on my left wrist early this afternoon.