The particular asana when rash really came strongly was dvikonasana and another one that I don't remember its name but it was similar - you interlace your fingers and then bow in your hips and put hands in front of you and you move left and right.
Well the rush is gone now
I had weak flow of energy or density in the area that asana stimulated and it manifested as a rash when the energy was releasing. (shoulders particularly) but it's all well now because it's smooth and almost cleaned now