Hard mass - lower left abdomen

Last year our hen Matilda had water belly and we drained her with success. She bounced back and has been doing well for several months.

Over the last few weeks however she has become lethargic again and has slowed down. We have examined her and found that her abdomen is a little swollen in general, but more importantly there seems to be a large, hard mass on the lower left side. From the outside it feels almost like an egg, but it’s not an egg. It has been there for over a week of examination.

She has no symptoms other than being a bit lethargic and having this mass in her abdomen. She is eating and drinking and ranging with the flock. She is 3 years old.

Any ideas on what it could be? We have started her on antibiotics in case it’s an infection, though I would have thought that if it was, she’d be way sicker or dead by now.