thanks for the query.
I would require more history to even suggest a differential diagnosis,but for now it seems like you are having an allergic rash but i am purely guessing, so please consult a dermatologist and get yourself examined and treated.
Wish you good health.
Such a rash which is present all over the body can be due to viral infection or allergic reaction. Hence it looks like a viral infection which is causing these symptoms. Please get an examination done from a pediatrician to confirm the diagnosis. Till then apply calamine lotion on the itchy rash and take oral antihistaminic like Benadryl for itching.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.