The exact same thing happened to my friends mare. Unfortunately, it was my mare that kicked her. We did call the vet and the swelling is fluid, mainly blood. It takes time for it to drain and will shift from one spot to another, usually from the site of the kick to underneath the belly, until it has been absorbed back into the body. It does take a long time to go away. I even called the vet twice because I was so concerned since my horse was the one that caused it. There is really nothing that you can do to speed up the healing process. He said that other vets would suggest hot packing it, but he did not think that would make a difference. Exercise is good for her as it keeps the blood moving.
I would suggest calling the vet just so you know for sure that there are no other issues.