Possible severe allergic reaction to metacam.

[Blobbyweeble]

My 6 month old puppy has had what appears to be a severe allergic reaction to a metacam injection (he's hurt his shoulder), he collapsed in the vets, screaming, became unresponsive and pooed and wet himself. His gums and ears went white, absolutely terrifying.
Has anyone elses dog had a reaction like this? The vet seems to think its very rare. I've left him there for observation after treatment with an antihistamine injection and oxygen he was perking up a bit.