Hello Sudden Rash Appeared On Lower Back Of High Leg,

hello sudden rash appeared on lower back of high leg, visually began looking like a nettle sting (i have not been anywhere near nettles) was slightly itchy.
As healing it turned into several pimple like spots and as faded has turned a darker red.
I want to know what it is? I have never had anything like this before and I have no illness or diseases. (Age 26)