I would say Ascites is a symptom of Egg Yolk Peritonitis if that's what your vet thinks she's suffering from. Reproductive disorders like EYP, cancer, tumors and Internal Laying can all have very similar symptoms and be accompanied by fluid in the abdomen (Ascites/water belly). Ascites can also be seen with organ dysfunction/failure. Often times it is not known exactly what to "name" the condition until the bird has expired and a necropsy is performed. It's good that they were able to drain the fluid and give you antibiotics, that probably made her feel better. There's less pressure on the internal organs and respiratory system. Draining is not a cure, but sometimes it does help prolong life and give relief. Sometimes draining may need to be done again after a while.