What Causes Chest, Trunk, Neck And Back Rashes With Itching?

I have a rash on my chest that appeared about 10 days ago. It started in one area and has now spread all over my trunk and back. I just noticed this morning that it is spreading to areas south and to my neck. I am afraid of what this is. I have a son that has a heart condition and doesnt need this. It started itching about 6 days ago and its getting worse. I have tried creams and benedryl. Nothing seems to make it go away. Any ideas of what this may be?. I have been getting pretty severe headaches since the started. Not sure if it is related?.