If she was getting free choice or all of her hay ration in alfalfa hay, then she may have been getting more protein than she needs and her body can't handle the excess protein, so her hind legs stock up.
Sorry, I don't know the exact medical names, etc, but I'm pretty sure that it sounds like what is happening with your filly.
As for helping to get the swelling to come down, cold hosing won't help this situation. Taking her off the alfalfa hay is the best start, maybe make sure if she is getting any additional feed that it is low protein for now, until the swelling is under control. I think that getting her circulation going will help. Lots of turnout time, rather than standing in a stall. If you already have her on lots of turnout, or even on 24/7 turnout that's great. Maybe, take her for walks on the lead a few times each day. If she was older I would suggest lunging and/or riding to get her moving and her blood pumping, but since she is only 1 1/2 years old, just take her out for walks in hand as much as you can. Getting her moving will really help to take the last of the swelling down.