Child Woke Up With Diarrhea, Vomiting, Developed Rash On Arms, Spreading To Cheek. Could This Be Allergy ?

hi there wednes early hours my 2 yr old son, woke up with diahorea and vomiting, also developed a rash on his right arm, thursday morn his arm had cleared up, but the rash developed on his right cheek, i freaked out and took him to our local gp, thay said it could be a allergic reaction or hives, may i add he didnt gve me a simple anser or ne thng to cure it, i went to the chemist and bought him some ANTHISAN< to stop the itching, after about a hour it clears that spot, but finds a new spot , its gone from, right arm, to right cheek, to left arm, to now friday morning, to behind his left leg, any tips to cure it up fast,, thay also said if after 24 hours after staartn the anthisan it hasnt cleared take him bac to the doctors, well after the first visit im not sure if theres any point, shall i takem to hospital????????? help please, and thank u in advance