Perhaps the issue could be lymphedema. The lymph system is similar to the cardiovascular system but doesn’t affect blood pressure. A blockage could cause a massive, rapid fluid weight gain.
I had a co-worker that suffered severe lymphedema after an auto crash. Her legs would become the size of tree trunks, skin taunt and painful. There is a massage method to reduce the swelling.
Lymphedema can be caused by morbid obesity as well.