What Causes Pink Welt On Thigh With Increased Temperature?

My daughter has a pink welt the size of a dime on her outer left thigh that appeared suddenly and has been present for a full day now. It is warm to the touch and has not changed in color or size. It does blanch when I touch it. It also seems to bother her when I touch it. She is irritable and feels warm as if she has a fever - which I had chalked up to the fact that she has molars coming in. Help.