Your cells are dying due to the excessive ultraviolet (UV) light exposure. However, these cells are within your skin so they need to be cleaned up from the inside.
The dying cells trigger the inflammation response which is part of the wound healing process. As part of the response, the local blood vessels around the sunburn open wider to allow more blood and other cleanup cells (white blood cells) into the area. More blood flow causes the area to be warmer and swell.
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