What Causes Whitish Blotches On The Arms, Legs And Foot Of A 3 Year Old?

My boyfriends daughter vomited last night a few times. She is nearly 3 years old. We noticed several silver dollar size blotches on her arms, legs, and foot this morning. She has no appetite, but has had a normal bowel movement and is urinating regularly. The blotches and bright red, non-blanchable and very hot to the touch. After her bath this evening, they became more itchy. They are slightly swollen, but flat and smooth. She has a fever of 102-103 (we are alternating tylenol and motrin and it has come down). She also is complaining of her throat hurting. Would benadryl be helpful, and is it safe to give benadryl to a child this small? Can we wait until Monday morning to bring her to the doctor. She has no respiratory symptoms, no cough, running nose, etc.