Hmmm...sometimes genetics do some crazy things. It's still possible she's part terrier and part pug. Just maybe not the kind of terrier you thought.
I don't really know anything about rat terriers. I don't think that the AKC recognizes them yet as a breed.
Let me see if I can find some pictures:
Rat terrier
According to their breed club web site, here is the standard for size:
With the body height being measured at the wither/top of the shoulder; dogs shall be shown in two sizes: MINIATURE, "10 inches, up to and including 13 inches" ; and, STANDARD, "over 13 inches up to and including 19 inches". Before 12 months of age, both sizes will compete together. At 12 months of age there is a class division for both Miniature and Standard varieties. Dogs not measuring 10" inches of height at 12 months of age, or measuring over 19" inches are disqualified from the ring.
Orchid is 16 inches tall and she weighs 35 lbs, and that is healthy for her. I'd say that that 20 lbs seems on the small side for a rat terrier according to this.