Can you post some pictures of the leg where the swelling is? is this area warmer than the other leg?
Also check the bottom of her feet like @HeritageFan recommended. Just to make sure nothing is going on there.
I have 3 Runner drakes and when they are young they are just like most young drakes very hormonal and will mate every chance they get. Most of the time my Runner females are after them to mate. And I have had a few girls come up lame every once in a while but never have they had a leg injury like your female it could be this drake or could she have injured her leg another way?
Also check the bottom of her feet like @HeritageFan recommended. Just to make sure nothing is going on there.
I have 3 Runner drakes and when they are young they are just like most young drakes very hormonal and will mate every chance they get. Most of the time my Runner females are after them to mate. And I have had a few girls come up lame every once in a while but never have they had a leg injury like your female it could be this drake or could she have injured her leg another way?