Did you happen to see her parents? She looks full chi to me. Many of the 'deer' type chi's (which is not correct according to the standard) are taller with very fine, thin legs and longer muzzles. They also have the tuck up at the waist and the slender features that many think of when they see a whippet or IG.
Brody has that same body type. He is 100% chihuahua. Just not a well bred one.
Then there is no way to tell if she is a mix or a purebred unless you want to do a DNA profile.
My guess is that the pet store saw that she wasn't the typical chi with the apple head and shorter legs and cobby body (which is specified by our standard) so they probably just labeled her as a mix to explain her longer muzzle, neck, and slender features (which resembles a whippet).
A DNA test might be fun to do though! Then you could know for sure if she was a mix or if she was a purebred chi.
:hello1:it could b a deer head.but,im not a pro n i dont have a dna test handy LOL,the first thing i thought of when i read your post n looked at the pics was that your pup definitely looks like he has a whippets nose.it just seemed alittle bit longer than your average deerhead.could b rong-just an opinion.heres my youngest baby-4 wks-in action.let me know if u have any name ideas