She isn't sore on it? then I would wait and give her a few days to see if it improves.
COLD HOSE It LOTS. Twice a day cold hose it for a good 20 mins. You said there was a bit of a laceration at close proximity, so chances are it seems to me she probably got kicked by another horse, maybe slipped and landed on something? got stung by a bug? who knows! these silly horses do things that are beyond are understanding.