What kind of mix is this dog?

The place where I adopted her said chihuahua mix, but she looks more like a toy fox terrier to me. The tail is especially thick like the fox terrier.
I adopted her Saturday at a Petsmart adoption. The place that had them there is not a very good place to adopt from, but I just had to rescue her. The man had her sitting atop other animal cages. She looked scared, like she was on a skyscraper. Poor thing.
He had her on heartworm preventative, but said nothing about having her tested for them.
Well, it turns out she has heartworms. Also she had an enlarged lymph node that I noticed when I picked her up and looked her over.My vet thinks she may have a tick borne disease and has her on antibiotics for three weeks at least until they can even think of treating her for heartworms.
She said there is no threat to my dogs since they are using Frontline and also are on heartworm medication year round.
She seems pretty lively, so hopefully this is something that is not too advanced. She is sweet as can be. The first time she saw Corkie our 80 lb. lab she squeeled from fear. She soon learned size does not necessarily mean vicious. LOL Corkie has taken to protecting her from any harm and she hangs out with her. I caught her playing with the cats today. I suppose they are more her size. LOL
http://www.PictureTrail.com/gid6730321
She only weighed 8.7 lbs when I got her. I can see she has gained weight already, but should not be much bigger. She is guessed to be about 4 years old.