Weird sunburn

Hi, I’m out in Palm Springs, CA. I’m British, so it’s been a while since I’ve been out in sun as piwerful as this. I was safe and wire factor 50 sun screen and spent less than an hour in the sun. When I went back inside, the sunburn was only on my chest and right shoulder, but was in the shape of all of what seemed to be my blood vessels on my chest. 10 minutes on and it seems to have subsided, however, the pattern is still there. Any ideas what this could be? This has never happened before and I usually tan just fine. Thanks. 

Here is a picture of said sunburn. 

Hello. That doesn't look like sunburn to me, it's more like some sort of rash or hives? You say it is subsiding now? Whilst it doesn't look like a heat rash per se, it may have been something to do with the heat. If it happens again or remains visible it might be worth seeing a doctor?

Hi, yes that makes a lot of sense. It completely disappeared about 15 minutes ago. It has been a very long time since I’ve been in UV levels like this, so a rash or hives is understandable. Thank you. 

Are you by any chance on any antibiotics or other meds that might react with the sun?