What Causes Scattered Bumps On Legs And Arms?

My 1 year old daughter started to get tiny red scattered bumps on her legs and arms two days ago. I figured that she might have had an allergy to some weeds outside because she was playing outside with short pants and short sleeves. I put some cortizone cream on her legs and arms and although she still has scattered bumps on her legs and arm, she seemed fine and I was just closely monitoring her. This morning she woke up with a bigger rash under one of her armpits and all around it and the bumps are still scattered on her arms and legs. She doesn t seem to be uncomfortable and is sleeping and eating well but I am obviously concerned. I would appreciate any help. Thank you.