here's a link that discusses allergies
Yorkie Dogs Health Issues - Allergies
Does she have any sores? There are several reasons she could be losing hair...eczema, ringworm...etc..
But most common are food allergies....especially to lower rated dog foods...even if she's been eating the same food for a long time..dogs can develop allergies.. Here's a dog food ratings guide...look up the food you feed and see how it rates... highest quality is 5 stars.
Dog Food Reviews | Dog Food Ratings