6 month old kitten ran away and was at humane society 2 days, has been home 4 now and slowly has stopped eat?

and drinking....when outside he runs and plays with his sister,climbs trees but when inside sleeps and is lethargic, now has started throwing up....i plan to call the vet in the morning but seems weird that this happened to a very healthy kitten...he had an additional distemper vaccine at the vet but not sure if that is what caused this