I've gotten that a few times and I'm pretty sure it was from the heat. The worst was when I went for a run mid-afternoon when it was about 30 degrees. My poor tummy, chest and legs broke out into that kind of blistery rash. Took almost 2 weeks to clear up! (I now stick to the mornings and evenings for my workouts without an issue) I don't think it was necessarily the sun that did it, but rather the fact that I was sweating so much. I've learnt that it's best to keep the skin as sweat-free as I can in the summer, especially the with feet(I live in a humid area as well). Maybe try rubbing baby powder on your feet before you put your socks on? Or you can try changing your footwear. I only wear sandals in the summer to prevent sweat build-up. At work, I've stopped wearing socks with my shoes and guess what? No more blisters! You can also try those non-cotton, moister-wicking athletic socks. I find that those really help.

I hope this helps!
-L