I have travelled a few times to hot & humid countries and always develop red blotches on the lower legs. They become very hot and it feels like they are burning from the inside. In extreme cases I have needed to apply cold gel packs to enable me to sleep comfortably. I always wear long pants or ankle length skirts, so my legs do not get sun burnt. The blotches are not raised at all, and tend to disappear within a couple of weeks of returning home.
I would like to know what causes it and how to treat it. I don't seem to get the same reaction when travelling to colder climates. I have posted a photo on my profile page.
Since you get these blotches only while travelling to hot climate, it can be a fungal infection, heat urticaria (rashes develop due to heat), contact dermatitis due to something your legs come in contact with (say while trekking). Do discuss this with your doctor and get yourself examined when you return with the rash. Take care!