Hi, my 4 year old girl Flora has just gone through this very thing although of course it may have a different cause. The important bit is that Flora is just finishing her course of Septrin for the URI and she is also on Frusol, twice a day, 0.3ml of 40mg/5 ml each time. This is about 2.4mg each dose, and she's just under 1.1kilo.
Flora had an ultrasound scan on her heart/lungs a week ago and the vet could see a small amount of fluid build-up. However she couldn't see any heart issues - no enlarged chambers or anything. It just looked normal to them. Even after careful listening with the stethoscope the vet could only hear a very quiet wheeze. I'd taken her in because her breathing was faster than usual at about 90 BrPM and she had a subtle 'head-bob' with each breath, but there was no noise. When she emerged from her tunnel (which wasn't often) you could see her sides heave like your girl. We tried just the Frusol at first which seemed to slow the BrPM but we still had the head-bob and the heaving... and at this point her breathing slowed to about 65 but it looked 'too' slow and she was clearly struggling. Her eyes were very wide and staring you could see the whites. But once we started the Septrin too she started to improve properly within 24-28hours, so there was definitely an infection. I actually tried dropping the Frusol for a day after she was improving but she was clearly worse without it, so then we reintroduced it and she's much better. Her breathing is much calmer and stays about 75BrPM at rest: no head-bob and gently moving sides. We may tweak the frusol, I'm also considering Furosemide tablets as an alternative if she doesn't settle on it, it has 10% ethanol in the solution it seems so it's going to be like a gobful of wine and she's not actually keen on it so I have to catch her (Flora is pretty cooperative generally and will just take her Septrin while she's standing there).
Flora had been spayed within the past month (v large ovarian cysts) and developed a URI straight after so was put on a short course of antibiotics then - but we felt there was something else going on as she'd gone down so fast after her op. There had been several instances of loud breathing over the preceding months: either 'clicking' or honking, or just strained gasping sounds, but each time it was just between a few seconds and a few minutes so we struggled to catch it on camera. Her other behaviour seemed normal and the vet could hear nothing with her lungs. But it looks like this has been brewing for a while. Another poster has a 'heart' piggy who has stayed on her diuretic but also has a heart medication called benazepet and this has stabilised her condition and helped her to regain weight - she's doing OK now. My girl used to be heavier but her ovaries were massive so I'm not sure how much weight she 'should' have lost with the spay, if you see what I mean. Right now she's maintaining OK in that she doesn't need syringing but she's not eating hay like she was. It may be her heart in the end, or it may turn out to be a mass somewhere (vet says this can also happen).
So whatever Flora's underlying issue it's causing this accumulation of fluid and that seems to be making her more prone to lung infections which will need to be caught as they happen. Hopefully by controlling the fluid and keeping a course of AB in reserve for quick treatment she will continue to manage until if/when things take a turn for the worse. I'm keeping an open mind as to the cause being heart, a growth, or something else. If 2 of your girls have the same thing but it's affecting Custard worse it does seem to indicate an underlying problem with her breathing / heart too.
How long had she been on antibiotics? They should take a day or two to start helping with symptoms - you can use your other girl as a measure of improvement. And what is her dose of furosemide?
