Call the vet if you haven't already. I don't know how old that filly is, but I can tell you that before I bought my old mare, Lady, she had a foal at her previous owner's and that colt was kicked by another horse in the chest - major swelling that the woman left untreated. She ended up with a dead colt. I wouldn't be surprised if that was the case here, too. Doesn't always look bad, but ANY major swelling means a vet call. Be sure to keep everyone informed on what is going on with her. I've got my fingers crossed for the both of you!
Ask Often, Demand Nothing, and Reward Generously.