Stained/Splotchy top of hands?

Hi there-
a couple weeks ago after working outside all day I noticed my left hand (top, not palm) had scattered splotchy pigmentation all over- like a dark brown/mauve stain or bruising.  It does not appear to be "typical" sun damage or sun spots, it has not faded in these 2 weeks, & I've noticed when I exercise, sometimes the skin will swell/bubble slightly with what I assume is sweat building up under the discoloration.  It does not hurt or itch, but I was curious if anyone knew what this could be.  I assumed after 2 weeks it would have sloughed off or faded, but no such luck.  I tried to add a picture of the worst part between 2 fingers and along the edge of my hand, but they didn't show up too well.  Any ideas or suggestions for diagnosing or treating?
Thanks!