I have had this...about 30 years ago. I got a really bad sunburn while I was flat-tar roofing in Montana. It was overcast, so dummy me took my shirt off. The sunburn was gone in a couple of days, but the pain was exactly as described: a thousand pin pricks firing at different times all over my back. My body twitched all the time, and the only relief was to stand in a cold shower, or lie in a cold bath with ice cubes. It lasted almost a week. I never did that again. In fact, I stay out of the sun as much as possible, except 30 minutes or more to get vitamin D.