Welco to Yorkie Ownership! There is nothing quite like it they are completely and utterly precious. This sounds simplistic but make sure she does not have fleas....if that is not the problem then remember yorkies have very delicate skin. You might give the clothes a break until you figure out the problem or get the scratching under control. I have considered giving my yorkie benadryl also because she does scratch after being bathed. I only bathe her once week now. The sprays to "sprooze" her up in the mean time do not seemto bother her. I would not give your yorkie anything until you ask a vet. My yorkie is tiny like yours and you need to make sure you give the right dose. You need to think would your 2 pound newborn infant benadryl?????
I think a safer bet would be do maybe apply a tiny amount of the liquid benadryl they have out now (it is the clear liquid gel in a blue and white bottle). I do know tha vets give antihistamines to puppies for allergies but I I dont know what kind or how much. This is such an exciting time for you-I am jealous! Please let me know if I can help because I have done many emergency trips to the vet for typical yorkie behaviors....I would suggest getting a boo on yorkies if you havent already and also read about reverse sneezing before you have an unnecessary vet bill. Hopethis helps.