Definitely sounds like waterbelly.
Eggbound hens will be straining to expel something... waterbelly hens have built up fluids in their abdominal cavities either from some variety of laying disorder or organ failure.
Based on what you're describing, I think she would benefit from being drained. That extra fluid makes digesting food more difficult, it makes breathing more difficult, and it's a lot of extra weight to carry too - which doubles up the strain on the heart.
Use either a 1" long 16 gauge or 14 gauge needle. These can be found at farm supply stores near the cattle vaccination stuff. The video starts with using a syringe to draw fluid out of the abdomen. Because I'm usually doing this on my own I adapted the technique to just poking the needle into the lower right abdomen and letting it run out using gravity, which is also demonstrated towards the end of the video linked below. Sometimes the hen will even continue to drip out the fluid after the needle is removed. How she responds will have everything to do with how she's feeling otherwise. Many times I've had them perk right back up if not immediately, by the next morning. If they're already on their way "down" draining their fluid is still a benefit because if they get to a point where they're weak and laying down, it becomes so hard for them to breathe- they literally gasp for air which is horrible.
Anyways, here's the video link: