Is the lump is above the vent or below it? Maybe it is not a distended belly but a tumor or an abscess?
My 7-year old Delaware got a big lump below her vent. I noticed it when I picked her up & saw this big ugly red lump a few inches below her vent. As GreedyGuts is my oldest and dearest hen, I rushed her to the vet, afraid it was an abscess or worse. The vet examined GG, stuck a needle in the tumor & drew out some oily liquid. Cytology showed it to be a fatty tumor & nothing to be worried about. If it got real big, I could have the vet remove it, but if it didn't impair GG's movement or life, then it could just be left as-is. Hers was the size of a golf ball, though, and you could feel it was a big growth just in the skin & it didn't involve the muscle below. Removal, had I wanted it, would be a relatively simple surgery with a great chance for full recovery. So not all lumps are that bad...
Abscesses are infections within the layers of the skin. They can appear suddenly and are usually painful. Once diagnosed though, they can be relatively simple to treat: usually they are lanced and the bird takes a course of antibiotics and the abscess goes away.
About Belle's lump:
When you touch the lump, does it feel substantial? Rock-hard? Liquidy?
Does touching it appear to cause her pain?
Is it hot to the touch, or red, or purple?
Does it appear to have a center, or is it uniform in surface texture?
How long has she had the lump? Did it appear suddenly, or swell up over time?
Does it stink?
Does it feel confined to the skin (can you slide it a little over the muscle below) or does it feel one with the underlying muscle?
Is there a vet near you who will see Belle?