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. The skin sometimes peels later just like a sunburn.
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.
These can be due to solar dermatitis or cholinergic urticaria. You can apply some calamine lotion at the rash as it will help in soothing the skin. You can take some OTC oral antihistamine medications like Benadryl or Claritin and see if it helps.
For mild-moderate symptoms a weak steroid may be used whilst more severe cases require a higher-potency steroid. But they are available under prescription and should be taken only after consulting a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.