It's due to the inflammation going on, I think. Sunburnt skin should have more local microcirculation to repair the damage. More blood in that area = more heat emitted. It is the same mechanism as when we are hot in the summer and the ends of our limbs turn red. Our body sends more blood to the extremities to more easily disperse the heat.
The sun is good for us, skin cancer exploded after the mass adoption of sunscreen. Gradually increase your exposure. Don't spend 10 hours in the sun after being cooped up in your house all winter. Start with 20 minutes, gradually increase this over the course of a couple weeks and you'll be able to spend a few hours in the sun without getting burnt. If your going to be out in the sun for hours upon hours, find some shade or bring shade with you..
Don't rely on sunscreen, it's absolutely poison.
https://thetruthaboutcancer.com/causes-of-skin-cancer/