If she has a club foot and the swelling is on the club foot or the opposit foot, then the club could be causing her issues. I would talk to your farrier. If he/she doesn't have any advice, find another farrier that has experience treating club feet. With the right shoeing (yes, will likely need shoes), stretching, and exercise, you can return a mildly club foot to normal or help make a moderate club move more normally.
If the swelling comes and goes it's not likely an infection. If the xrays don't show any joint or bone issues, then it's likely a hoof issue or soft tissue injury. If the farriers can't help, then I'd probably give her 2-3 months time off with hand-walking and some MSM and if it will resolve on it's own.
You want to get this issue addressed before it turns to something more serious like sidebone or ring bone. I would not continue riding the mare until you can figure it out or give her time off to let whatever is wrong heal.