swollen wattle

I noticed earlier that one of the girls has a swollen wattle. It's only one side and, having finally caught her, I can't see any obvious cause, although it is very warm. I wondered if it could be the result of a sting, but she doesn't seem bothered by it, still eating & drinking, and otherwise healthy.

If it is a sting has anyone got any advice on how to deal with it, please?

Also, any other ideas or similar experiences would be helpful. Thanks.

I've checked her over very carefully, can't see a puncture wound or anything remotely like one, not even a raised area. Looking at her from the front it's very definitely only one side, although the swelling has crept slightly across the area on her throat but is not affecting the left wattle at all.