I'm thinking that your cat has just "expressed" her own anal gland(s). Dogs and cats both have them. If this is the case, it is not a big rush to get to the vet, as they usually empty themselves naturally when the cat has a poop. This is likely what has happened as you say the cat seems in fine spirits otherwise. You can tell for sure it's the glands if it smelled so bad that you almost fainted
Here's a blurb on glands:
http://cats.about.com/cs/healthissues/a/analglands.htm