Does it itch?

I agree that the place to start is a different sunscreen.

As a separate experiment, try an antihistimine (Zyrtec, Claritin or Allegra) and see if that makes the rash go away faster.

You might also find it helpful to search some reputable sites for info on skin rashes, hives, angioedema. Such as this one:

http://www.mayoclinic.com/health/hiv...SECTION=causes

Ultimately you might find it helpful to invest in cycling clothes that cover more of your legs in hot weather, such as DeSoto leg coolers or Terry's summer knickers.