Hi everyone. About a month ago I purchased an Apple Watch (sport version, aluminum) and started having pain in my hand/wrist as well.
My reason: the aftermarket metal band. I purchased an aftermarket metal band and there is something about it (the weight, the way it hits my wrist, etc.) caused me pain.
My solution: the original sport band. I wear it snug enough that the watch stays in place while running, but still a little loose.
After several days of wearing the aftermarket band I was really having some pain myself. I did some reading on this forum and was concerned that I would have to take the watch back. I also considered the electronics were causing the issue. I adjusted the aftermarket band every which way possible. Thought maybe the raised underside of the watch (where the heart rate monitor sits) was causing a pressure point. Since going back to the rubber sport band I have been pain free and loving the Watch.
My conclusion: for some people, the problem is physical structure of the watch, band and/or weight. In my case, it had to do with the way the aftermarket band was hitting my wrist and/or the extra weight from the band was creating pressure points from the watch. I don't think it has anything to do with the electronics because I have gone back and forth several times between the original sport band and the aftermarket metal one, and the problem is obviously related to the band and/or its weight. Remember, the Apple Watch (face only) is almost double the weight as the Apple Watch Sport (face only). I think some people with problems should try out a sport version or at least a sport band. I can see something like the magnetic Milanese loop gradually getting tighter and tighter around someone's wrist as they move.
About me: Previously I have worn a Fossil metal watch band/watch for over a decade with zero problems. Very athletic, runner, low body fat, and would say that I have bony wrists.