It can help in diagnosis, at least with our cow ponies here in the western U.S., if you'll obtain the foal's temperature and herat rate. The temperature can be taken rectally and should be between about 99.7 and 101.4 degrees F. Take the temperature twice... once in the morning and once in the late afternoon to preclude the possibility of biphasic fever (a spike in temp later in day, your Vet would want to know that). A word of caution... the standard rectal thermometer used on humans is rather short in length. It will work just fine, but don't lose it in the horse! Hold it snugly with a rubber gloved hand (not vinyl, they're too slick).
Take the horses pulse rate. I find it easiest to locate a good pulse just beneath the jaw in the soft area. It should be around 40 to 45 beats per minute. You could also try taping the thermometer to the swollen area of the leg to see if it's hot. If you do this cover the thermometer with a clean cloth on the side not taped to the horse. It'll give you an approximation to see if the swelling is infected.
If walking doesn't appear to hurt her, be sure to exercise her a couple of times a day for at least 30 minutes each time.
My guess is she may have gotten kicked. But that's just a guess... Best of luck!