Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Samantha's weakness, lethargy and 2 days of diarrhea.
Diarrhea in an adult dog may cause some painful abdominal cramping and if it is very severe that can lead to dehydration and electrolyte imbalances. But most healthy adult dogs don't get weak due to simple diarrhea.
With her symptoms, sex and breed I am concerned that she may be suffering from Addison's disease. Addison's disease, which is a poorly functioning adrenal gland, can lead to lots of vague symptoms including weakness, loss of appetite and diarrhea and vomiting. Dogs may appear fairly normal on examination and on routine blood tests, other then showing signs of dehydration, so they can be difficult to diagnose. Some dogs will show a characteristic change in electrolyte patterns, and that is helpful in pointing us in the right direction.
These dogs cannot handle stress at all because their adrenal gland doesn't produce cortisone when stressed and their electrolytes can be off too if their adrenal gland isn't controlling that normally either. We can see physical collapse in severely affected dogs.
Standard poodles as a breed have a much higher incidence of this disease and it is more commonly seen in female dogs.
Testing is an ACTH response test to check adrenal gland function and checking electrolyte levels.
Treatment is steroid replacement therapy and electrolyte replacement.
Many dogs respond very well to fluid and electrolyte therapy, but will relapse quickly if proper medical treatment doesn't follow.
If you want to read more about Addison's disease here is a link to a webpage with accurate information: http://www.vetmed.wsu.edu/cliented/addisons.aspx
Another cause for sudden lethargy and weakness is a bleeding abdominal tumor. If her gum and tongue color is very pale/white that can indicate abdominal bleeding, and that is an emergency.
I know it is evening but ideally she needs emergency care. If she gets very weak I am concerned she may be very difficult to treat if we wait much longer to start therapy.
If you don't know of any emergency clinics in your area here are two links to help you find one:
I know that you are likely looking for home treatment but I am very, very concerned about your girl given her lethargy and I think she would be best served with immediate care that you cannot possibly give her at home, including intravenous fluids.
Symptoms of bloat (gastric dilatation with or without volvulus or twisting of the stomach axis) may include restlessness, a tight, rapidly increasing in size abdomen filled with gas, unproductive episodes of attempted vomiting, pacing, panting and sudden death.
These pups aren't able to burp, the gas stays in the stomach causing it to stretch and expand quickly.
Bloat is most commonly seen is large, deep chested dogs such as German Shepherds, Rottweilers, Greyhounds Great Danes, and Labrador or Golden Retrievers.
If she hasn't been attempting to vomit and her abdomen doesn't look like it is rapidly expanding bloat is unlikely.
Please let me know if you have any further questions.