Grandson, 6 Years Old, Had Unexpected ,seeming Out Of Blue

Grandson, 6 years old, had unexpected ,seeming out of blue at school yesterday, mainly on inner thighs, arms legs, cheeks, itching and very red and bothersome. Sent home, bath with same as usual products, same washed clothes, all organic, no chemical environment in home, 99 per cent sure nothing new eaten, except new organic oatmeal, but has eaten that past few days. After Benydry, he slept through night except late into sleep was awoken with the irratation. And then Bingo! Nothing this morning, all gone! Now what could possibly have been? Thank you very much for time and any info.