Sounds like a wasp sting, one of our pups got stung 2 days ago, around 4pm heard squealing coming from the garden & thought he had something stuck in his mouth but nothing, checked him over and was whimpering when I was touching his face, kept a check on him and by 8pm his right lip had swollen up, still managed to chew a bone mind

by morning he was fine, completely back to normal.
Just keep a check that the swelling doesn't continue / airways are all ok

Earlier in the day I'd seen a couple of wasps about, first time in donkeys! Wee blighters
