I really am sorry for your loss - it sounds like you tried so hard to help her. Its terrible when you just can't - I've been there enough times to know it all too well.
Before I get into possible issues there is a site called www.petloss.com
and the rainbow bridge story may bring you a little comfort like it does me.
Fluid build up can happen in the abdominal area for a number of reasons.
If the heart is not pumping as well as it might then fluid may build up instead of being pushed to the kidneys to be excreted as it normally is. Gravity and the more flexible open space of the abdomen makes it easy for fluid to accumulate there and the term for this is ascites.
Severe liver disease can also lead to this fluid build up. Obstruction of the hepatic vein or lower vena cava that leads fluid through the liver and to the heart can do it too.
Trauma can do it of course but you'd likely have known if that happened.
Disease of the lymphatic system can cause a build up of lymph or chyle in the abdominal area.
(Chyle is a mix of fatty compounds and lymph.)
But what I personally have experienced with my own dog was a mass in the body that leaked fluids into the chest and abdomen putting pressure on the heart, and lungs etc.
My dog actually had a nearly impossible to locate mass against his chest wall and in the heart area. It leaked fluids so that the first we knew of his illenss was the ascites and his shortness of breath because of the fluid build up.
Masses like this can also occur on the spleen and in other abdominal areas.
Sometimes what can happen is while the dog seems very ill from the pressure of the fluid build up that pressure also keeps the leakage down to a minimum. Relieving the pressure, which can give you more time with the dog in many cases by allowing better breathing and blood circulation etc, can sometimes also cause the fluid loss to accelerate leading to the body shutting down sometimes from blood loss sometimes electrolyte drop or fluid loss.
I was lucky with my dog as we were able to pull fluids off twice and make him short term more comfortable but the third time had to be done very quickly after the second time (indicating he was losing fluids faster) and we lost him shortly afterwards.
Based on your info on the effort you made you did all you could do. Its never fair to lose a dear pet and yet they just don't live as long as we do no matter how hard we try to keep them with us.
You may find, in time, that adding a new canine friend to your home, not as a replacement but as a new friend, will help some to fill the void your dear companion left in your life. While its never the same, and I think it shouldn't be, it can bring more happiness into your life as you enjoy the new dog and it brings back memories of the dear companion you lost.
If you really really need to know what the cause of death was and your dogs remains are still available a vet can do a necropsy which is like a human autopsy and sometimes that can determine cause of death for sure. Its an option if you feel that would bring you peace of mind.
Again my sympathies on your loss.