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.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles