Help diagnosing a skin rash

the dots left over aren't sore or itchy 

Do you know what provoked it? Were you unwell at the time?

Hi James

It doesn't look like a type of urticaria. I have dpu delayed pressure urticaria and chronic urticaria. This is not dpu as that is raised lumps on your body mainly from clothing that is tight or pressing on certain parts. Yours are red and scattered. Dpu is what the name implies - delayed pressure urticaria bought on by doing normal stuff that required some pressure and will come up as a few small wheals that develop into one large one and can itch like crazy. This is a very complex condition and there are so many different types and I am not an expert on rashes and hives I am only really familiar with what I go through. My advice is get it checked out by a gp. Good luck.

The don't look like my cold urticaria, which look more like white kettle rash bumps, like an insect bite, a nettle sting or sunburn blisters. But the rashes of different Urticarias look different.

Cholinergic (heat-triggered) Urticaria would make some sense in Vietnam.

But have you considered Shingles (have you had Chicken pox before). That can appear one one side of the torso. It could also be triggered by something else like:

Insect bites or stings (mites, flies, spiders, bees, wasps, mosquitos)

Fungal infections (ringworm)

There are also other autoimmune conditions that cause rashes like yours. For example, atopic Dermatitis. Have you been using suncream, repellent, different detergents, medicine creams or swimming / bathing somewhere that could cause this? 

I think you need to see a good dermatologist immunologist if it continues and is affecting your quality of life.