Same thing happened to my Sabrina, but I didn't notice it until it burst. :sad: I was so afriad because there was some blood in it and I thought she's pooping blood. I took her to the vet right away and the vet said he had to drain the pus and clean the wound or else the infection would become even worst. She seems fine ever since. The procedure is not a very long or serious one, but it does require sedation (unless the vet wants to risk his life!) The vet also said that this usually only happens in dogs but occasionally cats can have it too. Hope this helps
PS: Oh and the pus really really stinks!!! When Sabrina's sac burst, I thought the sewage system in my apartment burst!!!