My dog Daisy lost about 4 teeth at around the same age. It wasn't due to anything but her poor breeding. For peace of mind, I'd have your vet take a look but it's probably nothing. Good oral hygiene is important for yorkies so brushing her teeth is a good idea if you don't already do this.
Rawhides can have a lot of problems as can greenies so many of us here never feed these items to our yorkies. Many yorkie owners use bully sticks instead and feel they are safer. I no longer use them, but when I did, I bought them here:
www.westbrookdog.com