Update:
I just got back from the clinic recommended by my friend on facebook. This new vet obviously knows a hell of a lot more about snakes than the previous one. Another X-Ray was taken (which came out better than the last), but after looking at it, the vet said she couldn't tell what the mass was without an ultra-sound. To keep costs down, she did that for free. It turns out Shesha is full of fluid. A sample was taken out with a needle (which wasn't pleasant for anyone involved) and she ran some preliminary labs on it (used a "spinner"), which was also done for free.
The results on the fluid are inconclusive. There were a few red and white blood cells, but nothing that would indicate an infection, and there wasn't any "free floating" bacteria. So the vet isn't sure what it is or how it developed. She gave me an estimate for treatment, and to afford the surgery (which is necessary anyway), I can't meet the costs of a full diagnostics. If everything was done, it would come out to about $600, and it doesn't help that I wasted $145 or so on the last vet. Without the full diagnostics, it'll be about $450 or so, and she said she'd keep samples of the fluid so if I ever changed my mind on the lab testing, I could.
Shesha is still at the clinic undergoing surgery... sort of. They have to sedate her to remove as much fluid as possible using a needle and syringe. The vet is worried that she might need to be tube fed for a while to keep her strength up, but I said I'd try her on pinkie or small mice first to see if she eats on her own. The vet doesn't think she's overweight, just that the amount of fluid makes her seem that way. It's probably been slowly building up for months, and it just took a while to become really obvious.
I'll be picking her up later tonight. I don't think my husband will be too happy at the costs, but the vet was optimistic about recovery and it would break my heart to withhold medical care for an animal that could live a long and healthy life if treated. I suppose the biggest question at this point is where the fluid came from and how it built up the way it did. Otherwise this could repeat itself.