It is ventral edema. It is not uncommon before and right after foaling. It is especially prevalent in older mares, mares that have had several foals, mares that are low backed and have huge bellies, etc. More exercise helps a lot. If it does not start going down with exercise, she needs to be evaluated for more serious contributing factors like congestive heart failure and kidney function.
Keep an eye on her gums watching for any darkening of color and check her capillary refill by pressing on her gums (which should be pink) and then watching to see if it returns to pick quickly.
I would think long and hard about breeding her again. Once mares do this, they are usually worse with their next foal. Keeping her trim and making her get more exercise is the best way to fight it.