What Causes Red Welts On Arms Of A 2 Year Old?

Hi my two year old woke up from her nap today with red welts on both of her arms. It looks a lot like she was burned and they are irregular around the border and slightly raised this ones have now in the evening gone away only to be replaced with one large one on her rear end and on the back of her leg. They don't seem to bother her in the least. She was with a cold since last Tuesday. But it is clearing up now. Any thoughts ? Thank you