How To Treat A Burning, Itchy And Hot Rash Post Insect Bite?

I believe I was bitten by some kind of insect or spider. I didn't see what it was so I am not sure what to do. It happened yesterday, it was just a big raised spot about the size of a quarter. Today, it has gotten a rash around it, it burns, itches, and feels hot and hard. I don't feel very good either, what should i do?