What Causes Red, Itchy Skin On Arms And Chest?

My arms and chest are blotchy red and very itchy. It almost looks like a sunburn but I haven't been in the sun in a few days. There are no bumps or raised areas. What could this be? I am 26 years old, weight about 230lbs, and 5'8". 8 do have mitral valve prolapse. I currently do not take any medication.