Why Is My Body Rash Appearing Again And Again?

My two year old has been having an on agian off again rash for two days, it started out looking like a bug bite then started to spread My parents think it may be a allergic reaction. I have been applying calomine lotion and giving her benadryl. It goes away for about 4 hours but then re appears it is also accompanied with a small cough, is this something serious or is it actually an allergic reaction?