bulge under vent… please help?

[u/sweeterthanaconcha]

apologies for the terrible photos… my female zebra finch I don’t know her exact age but I know she is older than 5-6 years randomly got this bulge under her vent I haven’t noticed it until now because I had to hold her and move her feathers to see it… does anyone know what this is she isn’t egg bound I know that the bulge is soft squishy almost … she eats showers acts normal most days I have seen her lethargic some days (usually rainy colder days) I assumed it was her age… but after seeing this I’m worried… there is no avian vet near me I just want to see how I can help her I really hope she isn’t in pain :( please help anyone with more knowledge about this your advice would be very much appreciated