Are you an active person? Most people walk more miles than they realize in WDW. It could also be you are wearing shows that are not fitting you properly, not enough arch support and the heel box may be too small/short for you. Are you overweight?? Fat cells actually hold on to water. The more fat cells you have, the more water you retain. A diet high in sodium , sugar and carbs will make your body hold water. Tips for you: Eat lean protein, fruit, veggies and whole grains. Drink lots of water...not diet soda which has hidden sodium. Steer clear of things like popcorn, hot dogs, french fries... Tonga Toast is not a good idea for breakfast. I personally find WDW food to be WAAAYYYY too salty. At night cool your feet down and massage a peppermint foot lotion and lay on the floor or bed and rest your feet up against the wall and keep them there for 10-15 min and stretch your calf muscles out by pointing your toes forward and back.
For irritation on your heel, try Body Glide on your feet. It is a protective lotion that acts as a shield and prevents chafing, irritation and blisters. It sounds like your feet are swelling and getting too big for your shows and causing them to rub against the back of your shoes.
I am sensitive to sodium, and my feet swell when in WDW...and believe me I do over 10,000 steps a day - LOL. The combination of the highly salted food and the heat will make you retain water.