Is she acting normal? Laying eggs?
The swelling is soft and fairly pliable or tight like a drum or fluid filled?
Could be just a fat pad.
If it's tight and fluid filled, I'd monitor her for the time being since she's acting normal and active.
She's being plucked a bit on the bum, it's nothing major. If you wish, you can apply a bit of NuStock cream to the bare area.
Do look her over for lice/mites.
IF it's fluid filled, then you can try a
Detox to help reduce fluid.
IF it's fluid filled and she becomes lethargic, distressed, has difficulty breathing or shows signs of decline, then you can give supportive care by draining the fluid, it won't cure her, but can make her more comfortable.