I'm not sure but I get the exact same thing. It started about 2 weeks ago small and has come and gone twice already, growing in size each time. It seems the hotter I get the worse it is, not fun in 107 degree weather. Sweat seems to make it itch more, as well as direct sun. I dunno what this is other than torture! I thought at first it was sun rash that I used to get as a kid. However if it is, this is a severe form. Then I thought it was poison oak, but treatment did nothing. From the back of my knees down to my ankles, itchy and sore if touched with warm or hot water.