What Causes Purple Skin On Healing Of Sunburns?

Over a week ago I got severely sunburned on my legs and feet. I didn't blister but it was really painful. It's just starting to peel now, but the sunburned patches have gone a deep shade, almost purple. Normally, in the sun my skin just tans a bit. When I rub the affected areas they turn to a normal colour. This has only happened since the sunburn. What's wrong?