Could be some kind of abscess or a small prolapse of some kind. Can't tell without actually seeing it. The main thing is she still able to go poo? If the vent is blocked from the swelling, then it's quite urgent to get her in before Monday.
This sounds serious. If you can take your bird to an emergency clinic
that would be best. I know you posted this on April 6th. Today is April 20th.
Your post was some time ago. Hopefully, your bird is okay. Please let
us know how your bird is doing. Eve :)