I'll just share the few more common possibilities I can think of, that might cause a distended abdomen in rabbits.
Bloat from an intestinal or stomach blockage would be the first most common reason, but this is usually accompanied with severe pain and distress, along with eating and pooping completely stopping.
Next would be a gas filled stomach and/or intestines from enterotoxemia and severe disruption of the gut microflora, again causing distress and reduced appetite. This can be caused by an antibiotic, extreme carb/sugary overload in the diet, or picked up in the environment usually from another affected rabbit.
Sometimes a reaction to a particular food or veggie can cause gas as well, though not usually enough to cause a severely distended abdomen.
All these should show up on xrays as gas filed intestines/stomach.
If the intestines/stomach aren't showing up as filled with gas, then the likely cause is ascites from problems with the liver, or secondary liver problems caused by heart problems. With it being caused by heart problems, you're more likely to see other symptoms like fatigue and respiratory issues. With non heart related liver problems, you may not have any other apparent symptoms initially, other than the abdominal distention. Including no decrease to eating or activity levels until it progresses more.
This is something I saw in a new baby rabbit that came to my care. She had a distended pot belly, and was very undersize for her age. She didn't have any symptoms of lethargy, lack of appetite, or distress. Was pretty normal in every other way.
She had been fed a very low quality pellet, other high carb supplements, and little to no hay. I immediately changed her diet to a high quality pellet, but primarily free fed good grass hay. The pot belly went away, I think in the first week or two, and she started to catch up in weight and size to what was normal for her breed, and never had problems after (lived to be 10 yr).
Because she was showing immediate improvement with the diet change, no further tests were done, but it was suspected she either had a more moderate case of hepatic coccidiosis that resolved on its own, or liver problems related to the poor quality feed, possibly from mycotoxins in the feed.
This is what I would suspect is the most likely cause with your rabbit, a liver issue not caused by heart problems. Maybe from e. cuniculi, hepatic coccidiosis, or possibly mold in the hay. But of course it's just a guess. Your vet will have a better idea after the blood test results. But I would recommend double checking your rabbits hay and other foods for now, for any signs of mold or spoilage, in case that's the cause.
https://rabbit.org/health/liver-disease-in-rabbits/