Did you sell on a spay/neuter contract? Do you consider s/n before placing puppies? If the answer is no to either one or both questions, IMO, it seems that your buyer isn't the only one interested in making $$ off of puppies
To place pet yorkies w/o spaying/neutering or at least a s/n contract is not really being responsible in the placing your dogs.
There isn't much to be done about it now, except I would call her and try and educate her on the proper time to place puppies, etc.