This is 风疹. It looks like the “wind” which means it comes out very quickly and after a few hours, it may disappear as if nothing has happened. But it will come out again the next day or from some other place. It will spread very quickly if you keep scratching your body.
I had exactly the same symptom 3 years ago. Very terrible and very uncomfortable, and it lasted for 2 weeks. I tried all the possible methods including some Chinese medicines. Still not sure which medicine cured it or it just ended by itself.