We thought it might be a reaction to the preschool booster, but that was 6 days ago and if a child is to have a reaction to that it should be in the first 24h. So we think either a random insect bite or Contact with a substance or material - she was playing in a dressing up frock a friend lent us - quite nylonish fabric? Also a grow your own crystal kit but wondering if a nasty chemical caused it!? I have no idea but it was a huge and rapid spread - shoulder to elbow size. It's flattened out a bit now. Take care