What is the cause of an itchy rash that spreads throughout the body?

Hi doctor,

I am a 32-year-old female, and I work from home. Three days ago, I had an itchy rash on my right forearm. It has gradually gotten worse and spread to other parts of my body, including the top of my right arm (near my shoulder), my other arm, my inner thigh, my lower stomach, my lower back (which is where it is the worst right now) and it is gradually moving up my back. It is incredibly itchy. I have a severe allergy to poison ivy, but I know what it looks and feels like, and this just is not it. I have not been outdoors or changed my detergent or anything else in my household. I use the same cleaners and lotions. However, the rash on my right forearm is very linear, like a poison ivy rash would be. I have used calamine lotion, but I have not had any relief. I have been under more stress than normal lately. I have also had periods in my life where I was much more stressed than I am now. I am not on any over-the-counter or prescribed medications.