I am so sorry about your girl.
If she has a water balloon for an abdomen, generally the outcome is not a good one. Fluid is leaking into the internal cavities from possibly internal laying, liver disease or heart disease, there are no doubt other reasons, but these are the most common.
You can relieve her discomfort by draining her belly, however this is only temporary. The belly will refill. And what happens is, when the belly cavity is full, the fluid fills other cavities, eventually constricting the heart and the bird dies, if not from infection from internally laying or liver disease.
I have drained birds several times to keep them comfortable but all of them in the end, passed on.
You will need an 18 or 20 gauge needle and syringe, an alcohol wipe and a big towel to wrap up the bird. If you look closely at the skin on the belly, you should see pockets of yellow, these are where you want to drain. Other darker area will have blood, and you don't want blood.
You are going to wipe the area with the wipe and then stick the needle in 1/8 to 1/4 inch only. Suck out about 2ml. Then find another area and draw out another 2ml. Repeat this about 6 times. Never try to get it all out, you can send them into cardiac arrest. 6 times at 2ml each time is enough. She will drip dry over the next 24 hours. I use those puppy pee pads and keep them separated in a cage while they drip dry.
Just remember this will not fix the problem, only relieve her for a while. As to when she refills, could be 5 days, could be 2 months. Depends on what exacly is going on internally.