I just went thru this with my hen of about 18 months. It's ascites. There is a build up of fluid in her abdomen. I took my girl to the vet & he was able to drain alot of fluid from her abdomen. There is some people who treat for this at home & if you have a needle & syringe, you can too. I unfortunately can not buy a neele & syringe in MA so I had no choice but to take her to the vet. My hen was panting too when I first brought her in to the vet & the Dr had put her on antibiotics for 2 weeks, suspecting it was something respiratory related. when that didn't help, I was the one who came up w/ the ascities diagnosis & he agreed w/ me when I brought her in last week. If you google it, it mainly comes up as a broiler/meat bird issue, but it can happen in any bird. My vet says he has a parakeet that comes in every 2 weeks to be drained.
Hope this helps you out. Good luck w/ your hen.
Hope this helps you out. Good luck w/ your hen.