You may not experience this phenomenon, but here goes.
Say you’re wearing shorts to the ball game and you get a toasty little sunburn, but only from the mid-thigh down. Right at the edge of where your shorts were covering you, there’s a nice, clean transition from sunburned skin to not-sunburned skin.
Why is it that so much of the pain associated with the sunburn seems to happen right at that transition?
If you get a sunburn all over your back, for instance, the whole thing hurts plenty, but if you have an area like this on your arms or your thighs or somewhere, all the hurt seems to be concentrated in that twilight zone right at the edge of the burn.