It's that time of year, again - Mother Nature is waking up all of her creatures with longer photoperiods and warmer temperatures. Sounds like a simple spider bite, to me. HOWEVER; keep an eye on the color, diameter, and temperature of the welt. If the site grows a black point in the center, spreads quickly in diameter, or becomes an angry red and hot to the touch, you may want to be seen by a physician. MRSA has been confused (typically) as the bite of a brown recluse spider.
Otherwise, I would use a dab (a DAB) of Colloidal Silver TOPICALLY directly on the site daily until it disappears.