My 18 month old has a rash with runny nose. i have given him benadryl, (diphenhydramine) what could it be and should i take him to the doctor? he also has diarrhea.

Viral infection?: Good morning, rash, runny nose, and diarrhea are usually caused by a viral infection at this age. Most of these infections are self-limited and should get better with adjusting food - fluids such as pedialyte. Watch for any sign of dehydration such as dry tongue and sick-look. If the child looks, it needs to be seen by a doctor. Rash usually does not require any treatment.