Sophie started the same thing after we went on vacation to Florida. We left her at home with my dad and after about 3 days she started going everywhere. She had had maybe a weeks worth of accidents up to that point, now she has one every few days or sometimes every day! From what I understand, this can happen (and we didn't help matters when we left without her!). Yorkies can be hard to train, and just when you think you have, they do something crazy!
But she has gotten better and the accidents have diminished. So just keep working with your baby and she'll get it down again!