Hey there
This sounds like something I have too...I have a kinda dull red rash across the whole of my upper back and it especially flares up in hot weather or if I get nervous or flustered. I went to the dermotologist about it last week actually, they were rubbish

but I had a look online and I reckon it's something called keratosis pilaris. This is the same thing that causes little bumps on people's upper arms. If this is what it is with you guys it's genetic and there isn't any treatment for it, but apparently it's worse in adolescence and gets better and often clears up in adulthood. Sunlight helps it and exfoliating regularly will help too.
Hope this helps!
J*L