They are 2 different breeds completely, although the Silky Terrier was created by originally breeding Yorkies with Australian terriers.
American Kennel Club - Silky Terrier American Kennel Club - Yorkshire Terrier
I personally have a hard time telling the difference, though I've been told (and a lot of people here will vouch for) that if you see a really good example of each you can immediately tell the differences.
Now, it's possible that you do have a purebred yorkie girl. She just may not have been bred to standard. If that's the case, you should really reconsider breeding her. Breeding should be reserved for the betterment of the breed and isn't something to undertake lightly.
Out of curiosity, was she registered AKC or CKC? They have vastly different standards for proving you have a purebred pup.