I have had this rash for years and did not know what it was. It always appears after I have been out in the sun when it was very hot and humid. I thought it was due to the socks I was wearing with my tennis shoes so I got socks that did not go above the shoes. That did not stop the rash.
Now I again have it from sitting on a garden stool planting. It was very hot and humid Florida day. I looked down at my angles and there it was. It is
very ugly and somewhat painful to the touch. I just ignore it and after about two weeks it fades away. Once I have it on one leg, I usually have a mild case on the other. It also can be itchy.
I never had it when I was younger and I was always very active. I even get it when I ride my bike in the hot weather. I just grin and bear it. What else can you do.