Um actually I would suggest takin the dog to vet..my american bulldogs back legs swelled up && she lost control of her whole back end. It was due to blood clots && it didnt turn out well...she had to be put down so best suggestion would b to take your pup immediately to vet.
Many things from arthritis to an infection which is unlikely since it is both feet. You need to take your dog to the vet to have a check up done.