I agree - I would have her checked out and spay her. Yes, a sonogram would be able to check into it further. I would ask for the sonogram and see. You can also do a blood test to also check for her blood chem. levels. These are the things I would do. You asked and just reporting my expert opinion. I hope it helps. If Gidget was born Sept. 17, 2006 - and you have had 3 litters from her so far to date! I would not breed her now - no way!
I have most of my females go into heat about every 9 months too and I always rest them. I did some research and this is what most of the vets on-line are recommending. Here is one:
Yorkshire Terriers/Yorkies: yorkie breeding question, franny, litters
What exactly is a puppy agent for breeders - you advertise your services on your web page? Not a broker, right? Just making sure.