It could be a sweat rash.
Does the rash, weep, then crust over at all?
Have you touched anything, even accidentally, like any plants?
You may benefit from taking an antihistamine. Calamine lotion or calamine cream can help with the itching.
Keep well hydrated.
This same thing is happening to me!! I have no clue what mine is either. It is exactally like you are explaining. Mine started under my armpit, closer to my ribcage/stomach area, where my bra strap would be. They are all over my back, stomach, legs and arms. Mine start with a red area of skin and then some looked like they had a head on it. After a few days they turn into brown marks, almost like a small bruise. They stay like that for about a week or two then slowly fade away. It is very strange. I have always used the same soap, lotions, etc. years before this started. Recently I changed things up and stopped using all my old products and months later the same thing is still happening. I have moved out of state, and pretty much eat all the same foods as I used to. I can't figure out what mine is.