Rash on my arms for the past 2 days

Don't quite know what to make of the rash on both of my arms that I noticed around lunch time yesterday right after  I went for a 15 minutes walk around the building in the sun.

I'm pretty sure it's either a heat or sun rash.  The weird part is I don't recall ever having a sun rash. I'm not on any medication either.  I was reading on sun rashes, it happens to fair people, I'm quite fair, but usually stops by age 40, I'm nearly 50. 

Admittedly my arms were the most exposed to the sun last weekend, I wore tops that had very short sleeves. The arm that is the worst is my driving arm. But I did wear sunblock every day except Monday the day I was driving home.  The other thing I wore on my arms was bug spray in the evening, Citrobug.

So is it a reaction to the sun, the sunscreen, the bug repellent? Who knows! But it's the 2nd day I have a bumpy rash on the top of both arms, that makes them look sunburned, because it's reddish. 

Someone suggested that it could be stress... but then why just on my arms?

My arms itch and burn at this point and I try to ignore it as scratching or rubbing tends to make rashes worse, or turns little spots into hives. 

EDIT 10AM - It's definitely a sun rash. I noticed it's also on my neck front and back. This is mostly where I was exposed to sun yesterday in my walk, and rash got worse on left arm when I drove to work and it was in the sun the whole time.