Gail, know it could be because of some benign reason as well. Fluid retention has many different causes. Some are critical and some are not. Yes it could be the heart or the kidney. It could be something as simple as lack of exercise causing the edema. Mom had chronic consistent edema in her arms and legs for months and months... with no heart of kidney involvement. It was directly related to her excessive pacing. Now that she is resting more it has gone away. Also know that medications can also be the cause of this problem. Steroids, beta blockers, and a list of other medications can cause fluid retention. Insufficient protein as well as some other nutrients can also trigger fluid retention. Another frequent cause is a sluggish lymphatic system. Breast cancer survivors who have had lymph nodes removed often have edema in the related arm. The body is mostly water. It's a delicate balance between the capillaries and the cells to keep the right amount of fluids where they need to be. Any disruption in the cycle can cause edema and it is treated depending on the cause.
I hope it is something benign and she will respond well