Very bad rash! Help please?

Ok since last night I noticed a bumpy rash on the other side of my ear. This morning I woke up to find that it had spread to my left shoulder and neck. It's red, bumpy , itchy and it hurts alot. I haven't used any new products and I don't think I have any allergies. I also don't think they are bug bites. But based on what I have just said. What is this rash, how long is It going to stay, should I go to a doctor. And do I have to miss school today because of it (due to going to a doctor?) please answer I am kinda worried!