Our little Jasmijn has the same problem.
I do not know, is there also a dark/brown fluid coming out of it?
Jasmijn's problem is the following: with her sterilization, the vet
probably forgot a small peace of her ovarium. The next time she has a
swollen vulva, she has to be under surgery.
Rgds,
Thirza
[Posted in FML issue 3744]