Does it feel perfectly centered, like it's right over the middle of the bird's hips? Is it hard, or soft like a blister? Does it feel much warmer than the surrounding skin? Looks almost like a uropygial gland gone bad, or a surface wound with infection brewing.
A vet is your best bet. A vet who sees small animals specifically, if you can find one.
Until you can get her seen, I'd try holding a warm compress to the site (don't scrub, just press gently). If anything comes out of the wound during, gently wipe it away and take note of what it looks like. A thin smear of triple antibiotic ointment isn't out of order, either, but isn't likely to help much if this is an infection of deeper tissues.
Edit: and by "couldn't walk," do you mean having trouble walking, or completely unable to walk?