Hi and welcome to YT!
Smaller yorkies are more likely to have episodes like this with low blood sugar. There are several medical issues tiny yorkies are more likely to suffer....that's why it's generally not a good idea to promote buying, or breeding for them.
I would make certain she gets a good quality food and has it available to her at all times. Try to make certain she is eating several small meals during the day and evening until she is a little older and hopefully bigger. In the future when she won't eat the Nutrical, rub it on her teeth or gums, she will have to lick it off.
Yes, the breeder is aware that tiny yorkies are more likely to suffer medical problems...but they usually don't care (especially if they are selling to pet stores!).....it's all about the $$ for them!
And it's not unusual for them not to follow up with return phone calls, I'm sure.