It could be something called 'soremouth', which sheep and goats can easily get. It is contagious, and can get uncomfortable, but I've never seen it lethal. Here's a link to more info about it. It says it's not treated by antibiotics, but I have seen it successfully treated with a topical tetracycline spray, or with the use of a barrier cream on the lips & nose of the lambs and udders of the ewes.
soremouth info page