It might just be a piece of thistle or something from hay. That happened to a friend of mine, they got thistle from a new bale of hay and it became swollen and resembled a lump. Their vet came and lanced it with no problem (they were going to a show in a couple of weeks and any lump on the face/neck area, no matter what it is, is obviously not allowed to leave the trailer) and there wasn't anything wrong with it. Hopefully thats all it is!!