Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Meggie and her diarrhea and loss of appetite.
Diarrhea means that the gastrointestinal system is irritated. If her colon is affected you may see mucous or bright red blood in her stool, the darker the blood the more digested it is and the farther up the gastrointestinal tract it is from, you might also see more of a constant urge to go and small amounts of diarrhea frequently. If it is small bowel diarrhea you won't see mucous or bright red blood but can see more stool then usual and it will often be watery, although the frequency won't be much different then usual.
Diarrhea can be from a bacterial or viral infection, eating something she should not have (toys, bones, bugs, your other dog's food etc), gastrointestinal parasites, a dietary allergy or sensitivity, or a quick change in diet. Inflammatory bowel disease is possible but cancer is less likely as she is a bit young for that.
We can try some things at home but if she isn't feeling much better in 24 hours then I think a hands on examination is a great idea.
Because she has lost her appetite she may have some stomach upset and reflux that can go along with diarrhea.
To try and settle her stomach today you can try either:
1) Pepcid-ac (famotidine) at one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at one 10mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation. These are very safe medications and can be used as needed for several days if need be.
I would not offer her any food for 24 hours. This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as she needs fluids after all that she has lost with diarrhea. You can give her pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make her diarrhea worse.
Today even with the fast you can start Kao-pectate (rather then Pepto or Imodium) at 1ml per pound or 1 tablespoon per 15 pounds of body weight every 6 to 8 hours. This is quite safe and will coat her irritated gi tract as well as absorbing bacterial toxins. You can use it for several days until her diarrhea stops. You can find kao-pectate at the drug store.
If she has a tense painful abdomen, continues to have diarrhea with the stool not firming up after being on kao pectate for 24 hours, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then she really must be seen by a veterinarian as soon as possible.
After 24 hours start a bland diet of 1/3 boiled hamburger (or boiled, white skinless chicken), all fats and juices drained off mixed with 2/3 boiled, plain white rice. Feed several small meals a day.
Once she feels better (no diarrhea for 48 hours) start mixing in regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert her back.
You should submit a fresh stool to her veterinarian for a parasite check even if she seems to be feeling better. Parasites are a very common cause of diarrhea and can be shed episodically, so the symptoms can be on and off.
Let me know if you have any more questions.