What Causes Itchy Red Rashes Throughout Body Along With Nausea And Vomiting?

Since last night, my 2 year 10 month old daughter has developed an itchy red rash that welts up over her body...from her face, neck, torso, and extremities...she had nausea and vomiting on Thursday, we were thinking it was just a stomach virus that has been going around, because the N/V was accompanied with diarrhea and lethargy...we have gone over any foods she may have eaten and detergents and such; nothing new or alarming...any idea what may be the cause?