my moms dog did that same thing when i was watching them for the weekend..... i freaked out i didnt want her to come home and say i tried to kill her dog
..... we concluded that it had to be a wasp, there arent any really dangerous spiders around my parents house, but there are wasps a plenty........ it swelled up just like recons and lasted a week at least... she kept shaking her head and scratching at it, eventually she scratched a puncture in the effected area and it started to puss and blead out (sorry for the gross description...... but thats what it was)