I am sorry to hear you are worried about your terrier. My name is Rebecca. I am a veterinarian and will do my best to help.
Pale gums and lethargy are concerning to me. If her gums are very pale, she is probably anemic, her red blood cells are lower than normal. Anemia can cause weakness, lethargy, labored breathing, and eventually it can lead to shock or even unconsciousness.
Anemia is caused by many things: certain infections, such as tick and flea borne diseases, internal bleeding, autoimmune diseases, ingesting rat poison. Treatment of anemia is two pronged: 1. see how low the anemia is, and if a transfusion is needed to stabilize her, and 2. find the underlying cause and treat it. If it is rat poison, we treat with transfusion if needed, and vitamin K1; for autoimmue diseases we use steroids, for tick/flea borne illnesses usually doxycycline. If there is internal bleeding, we have to find the source of the blood (bleeding ulcer, bleeding tumor) and treat that.
I am very worried by your description of her. Can you take her to an emergency vet clinic?
Let me know what other questions I can answer and how she is doing.