5 yr old with red spot and pain in underarm

My son is 5 years old and has for 3 days had a red spot about 1.5" in diameter under his arm.  He has said its sore and did start out smaller. He has a doctor appointment tomorrow needless to say.  He had a virus that really kicked his butt about a week prior to this occuring.  The area isnt hard or swollen it just looks very red like a incerased blood flow to the area.  Being a dad who loves his son I am just trying to get input before the doc visit to have more potential questions to ask.  He hardly ever gets sick and it all boy, meaning he is rough lol.