I bet I have the answer. We experienced the same thing with my daughter at age 5. google ITP. Immune thrombocytopenic purpura; ITP
My daughter one day developed these red dots under the skin. They didn't blanch. (meaning when you press on them they dont disapear for that second.) For instance if you have a red pimple or mosquito bite and you press on it, for a second it will turn back to your normal skin color. We went to the pediatrican who said they were under the skin. Didn't blanch, she ran blood work. The only thing not good with the blood test was lower than normal platelet count. I quickly made an appoint ment with a Hematologist. (a pediatric one in Cornell in NY) We went, they ran blood again, and still a little low, he looked at her spots and was like I've never seen this before. So now off to a dermatologist. 3 for that matter. the 3rd one knew exactly what it was. He said it's schamberg's disease. which is a form of ITP. Google that was well.
No treatment needed in my daughter case. it is not gone she is 7. But can come back at any time.
Background on how my daughters looked. on her upper thigh, started small like the size of a dime. gradually spread, to aboutt he size of a fist. Got darker in color, kind of like a rust color. stayed that way for about a year. here and there she would develop, bright red tiny dots under her arm pits, they would last 2 days and go away. Seemed to only happen in the summer.
I hope this helps. I made myself crazy thinking horrible things, but the good news is, it went away, bad new is, it can come back. Good luck and keep me posted.