There's a special kind of dread that comes with finding a hard, worry-shaped lump on a hen you already saved once. You beat water belly. You drained her, watched her bounce back, and let yourself believe the hard part was over. Then Matilda slows down, you run your hand along her abdomen, and there it is: something that feels like an egg but absolutely is not. The internet is full of guesses, but what you really want is someone to say *I've seen this before and here's what you do next.*
Let's be honest: a hard, persistent mass in a hen's lower abdomen that doesn't respond to antibiotics and doesn't kill her quickly is not a comforting sign. You're right to notice that if it were a straightforward infection, she'd likely be much sicker by now. That's the thing about chickens, they hide their worst troubles until they can't. Matilda is still eating, drinking, and wandering with the flock, which means she's either tough as nails or the mass is growing slowly enough that she's adapted. Neither is a guarantee of good news.
What you're describing sounds like it could be a reproductive issue, which is sadly common in hens who've already had abdominal trouble. Hard masses in that spot often turn out to be a stuck yolk that calcified, a tumor, or an impacted oviduct. The fact that it's been there for over a week without her crashing is somewhat encouraging, active, fast-growing infections tend to make themselves known quickly, but it also means this isn't something that will drain away like the water belly did. If we were advising a reader, we'd say stop the antibiotics unless a vet confirmed infection. They're not going to dissolve a solid mass, and they can mess with her gut bacteria. Instead, get a hands-on exam from an avian vet if you can. That's the only way to know whether this is something you can manage with supportive care or whether it's time to consider quality of life.
Here's the concrete point: Matilda already gave you a miracle once. That doesn't mean she owes you another, but it does mean you know how to give her comfort. Focus on what you can control, soft food, easy access to water, a quiet spot to rest, and get a professional opinion before you make any decisions. However this ends, you'll know you didn't guess your way through it.