What type of reaction is this?

Different times in my life when I get hot I break out into small hives and get really red and splotchy. It itches like crazy! I thought it was nerve related, but it's happening again and I am not stressed or anxious about anything. I am also on anxiety medicine which should prevent anxiousness. I had my blood pressure checked along with complete blood work and all appears to be normal. I have also seen a dermatologist and tried various steroids and creams. I get hot and start to break out and itch like crazy. Once I cool down it goes away. It can last for 5-10 mins. I live in Texas so it is really hot all the time! This is very frustrating and all of the doctors I have seen are at a loss. Any idea what this could be?