What Causes Red, Blotchy Rashes Under Side Of Arms?

i have a red blotchy rash on the under side of my arms and red on the top almost looks like sunburn but the impossible have been indoors all day tops of arms warm 2 touch could it just be heat rash im 29 about 5f 4inch 90kgsstarting on my legs a wee bit 2 was working in warm enviroment 4 5hrs