It does sound very worrying, but it would fit with a infection, especially as it started just after a trip to the vets. It could have been a small virus she picked up there. Is she on chemo? A friends cat had to have a course of chemo and it does make them immuno supressed and open to viruses/infections that otherwise they could have just fought off. Even being a little bit run down would open her up to the same.

Unless there is a lot of blood, from what I have read some blood staining is common where the nasal passages have been inflammed for a while, people get the same after a heavy cold. I hope whatever antiBs she is on now does the trick. There is an antiB injection now that last two weeks, convenia or something like that.

I hope she continues to enjoy herself and this problem resolves itself very quickly.