I'd wash the leg well with a cloth and mild soap so you can see and check for splinters or any obvious break in the skin becoming infected. The other cause would probably be some kind of soft tissue or bone injury, both of which would probably benefit from cage rest and warmth (warmth really speeds healing). There are quite a few options for poultices, if you're so inclined. Comfrey and plantain are excellent for swellings, as well as many others. Good luck.