I'm far from being an expert but, It doesn't seem possible that a female yorkie mated with a male shepard.!! Did you actually see them
"TIED" together??
Last year when I tried to mate my female yorkie, I thought for sure she was pregnant I saw them tie and she seemed to be getting somewhat heavier, being very lazy, etc... But she was infact not pregnant.. Its not until after she stops bleeding that they are actually fertile.. So either bring her to the vet (all he can do at this time is feel for pups) or sit it out and wait a bit.. Either way good luck..