My son received, what appeared to be, a bug bit (small red bump) on his back (close to the arm pit) yesterday. It was itching him. So I applied cortisone to ease the itch. Tonight, I noticed it has grown to form a welt, and seems to be spreading underneath the armpit. The welt has the shape/size of a Lima bean, but the redness is spread out to be larger of an area (larger than a half dollar coin. What could this be?