Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am very sorry to hear about your pup Zoe's eating grass, and diarrhea.
Dogs with nausea or gastroesophageal acid reflux often eat grass or foreign material to make themselves vomit. They will often swallow repeatedly and lick things trying to soothe their nausea and acid burn. If they can rid themselves of whatever is irritating their stomach that way then all is good.
But when their stomach upset persists and they continue to be uncomfortable then we need to stop them from eating any more grass. It becomes a vicious cycle where the more they vomit the more it irritates their stomach and the worse they feel.
Possible causes of nausea and diarrhea causing grass eating include a change in diet, dietary sensitivities or allergies, or eating things that they should not like too many fatty table scraps or garbage, bones etc. Addison's which is a poorly functioning adrenal gland is another possibility for waxing and waning vomiting and nausea. If she's a younger pup metabolic organ failures (kidney or liver disease), pancreatitis, inflammatory bowel disease or even infiltrative cancers are possible causes but less likely.
If she doesn't get many table foods very often perhaps the tuna did trigger this.
You can give her acid reducers to try and settle her stomach. Either:
1) Pepcid-ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle her stomach. These can be used for several days if necessary as they are very safe.
I'd also pick up her food and water for now. A couple hours after an acid reducer is given you can offer small amounts of water or ice cubes to lick. She's likely thirsty but we need to settle her stomach first.
No food for 24 hours. Small amounts of water only.
Today even with the fast you can start Kao-pectate 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 stools look normal. You can find kao-pectate at the drug store. I like the kao-pectate formulation with kaolin and pectin better then Pepto Bismol as Pepto has salicylates which are in the same family as aspirin, and can further irritate the intestines.
After her 24 hour food fast then start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice. If she has a little blood or mucous in her stools add 1-2 tablespoon of canned pumpkin (not pie filling, just pumpkin) to each meal as fiber helps soothe an irritated colon. I recommend adding probiotics like Fortiflora, Proviable or Benebac to her meals daily to re-establish normal gut bacteria levels too. Give small meals several times a day. Feed the bland diet for several days, then start mixing in her regular diet and slowly convert her back.
Once she feels better (no diarrhea for 48 hours) start mixing in her regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert her back.
If her diarrhea or nausea continues then she may need injectable medication from her veterinarian to get her stomach upset under control.
Even if she seems to improve it is well worth checking at least a couple fresh stool samples too as parasite eggs/cysts are shed intermittently and they may be picked up on the second sample or third test if the first is negative.
Please let me know if you have any further questions.