Kidney problems and heart problems can both exhibit with stocking up. Excess protein requires that a horse drink more water to metabolize it through their kidneys. [It is excreted as ammonia in the urine.]
Usually hind legs stock up first, then front legs and finally lower abdomen. If lowering the amount of protein fed does not help, I would have a Vet check for congestive heart failure, heart valve problems or diminished kidney function.