What Causes Rashes All Over The Body Along With Severe Itching?

I had rashes all over my upper body when i woke up this morning and it was itching badbly so i took a single 25 mg tablet avil 25 mg and there was some relief. I took it at 1 pm in the afternoon and all the rashes dissappeared within 2-3 hours. But again at 6 pm rashes started to apperar again what should i do