rolo had this . it took 2 lots of antibiotics to clear . his was huge . then about a month later it came back. more anti bs.
the young vet I saw put the fear in me as he mentioned the nasties 1st. turned out to be infection tho.
how are lucys teeth and gums? as any disease there ie gingivitis can set this off.
Rolo had caught kennel cough as all my 3 did . they all got over it fine and Rolo was the only one to have glandular involvement which was weeks later but the only thing I could think of to set it off. as your vet said it might have been brewing for a while.
best of luck to you, I can still remember how scared I was.
