What Causes Rash On Stomach,chest And Back?

About 3-4 months ago I developed an oval shape large red spot. It didn't hurt or itch so i didn't think much of it. 4 weeks later, i woke with spost all over my stomach, chest and back.Ii ran to the doctor and i was told that it is a very common rain rash which will be present between 4-6 weeks. I wasn't given any medication. It cleared after about 4 weeks. I feel fine and don't seem to have any side effects. What is the medical term for this rash? Can you tell me more about it?