This, indeed, can be a heat rash, happening due to excessive sweating in hot weather.
Try to keep the area cold and apply some powder.
It is important to avoid sweating for few days and the rash will go away on its own.
If you had some viral infection recently, like cold sores, this rash can be triggered by the viral infection, and is known as erythem multiforme.
Best wishes!
Marija