I have an identical bruise! Not quite as bad as yours. I have exactly the same watch (38mm modern buckle) noticed it yesterday having had my watch for about a week. The buckle doesn't seem to touch my wrist there so I don't understand how it is causing it!
My bruise is gone now, and no new ones, but I've been very careful to keep my wrist up off of my laptop as much as possible. That's all I can figure out may have caused it -- bumping harder there than I noticed. If I wear my sport band it's not a problem, but I like the look of the modern buckle better so I wear it much more often than the sport band.