Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am very sorry to hear about Cooper vomiting yellow, eating grass, and having diarrhea with mucous (the phlegm like substance) and I'd like to help.
The yellow color to his vomit tells me that he has bile refluxing from his small intestine into his stomach which he is vomiting. While this isn't normal it is seen often with severe gastrointestinal upset which leads to abnormal gastrointestinal motility and bile reflux. I suspect that his vomiting was simply due to gastrointestinal upset from his whatever led to his diarrhea.
Mucous in his diarrhea indicates colitis, or inflammation of the large intestine, as only the colon produces mucous in response to irritation.
Dogs eat grass with an upset stomach, but at this point because his symptoms are lingering repeated grass eating will likely only further irritate his gastrointestinal tract, so you need to stop him from eating any more.
Vomiting and diarrhea can be related to something as simple as a quick change in diet or treats or getting into something he should not have like the garbage, a bug, plant material or a toxin. More serious causes are viral or bacterial infections, gastrointestinal parasites including worms and protozoal parasites, chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, internal organ failure, a full or partial gastrointestinal obstruction or even infiltrative cancers like lymphoma.
Is Cooper current on his vaccines?
Does he go to dog parks or doggie daycare or has he recently been boarded?
Is he a pup that likes to chew on and ingest things he shouldn't?
If he seems otherwise happy, bright, playful and alert as usual, those are good signs.
If he seems perfectly normal otherwise it is possible that he got into something that didn't agree with him and that caused stomach upset and irritation and diarrhea, so there are a few things we can try.
To try and settle his stomach today you can give an acid reducer, 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 quarter of a 20mg tablet per 10 to 20 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 medications can be used fro several days as needed as they are quite safe.
I would not feed him any food for 24 hours after the acid reducers are started.
This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as he needs fluids after all that he has lost with vomiting and diarrhea. You can give him pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his intestinal irritation worse. Do not allow him to drink too much at any one time as that can lead to vomiting.
Today even with the fast a couple hours after the acid reducers you can start Kao-pectate at a dose of 1/2 ml per pound or 1/2 tablespoon per 15 pounds of body weight every 8-12 hours. This will coat his irritated gastrointestinal tract as well as absorbing bacterial toxins. You can use it for several days until his stools look normal. You can find kao-pectate at the drug store.
After his food fast start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken), all fats drained off the meat, mixed with 2/3 boiled, plain white rice.
Since he is having mucous in his stool add 1-2 tablespoons of canned pumpkin (not pie filling, just pumpkin) to each meal. Fiber soothes an irritated colon.
You can add probiotics to replace normal gut bacteria which is often lost secondary to diarrhea. I recommend Fortiflora, Proviable, or Benebac.
Feed small meals frequently until you figure out that it is sitting well with him, then gradually increase the amount and decrease meal frequency. I would start with 1/2 cup for the first meal.
Once he feels better (no vomiting or diarrhea for 48 hours) start mixing in his regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert him back.
Well worth checking at least a couple fresh stool samples too even if he seems to improve as parasite eggs/cysts are shed intermittently and thus are not seen in every sample. They may be picked up on the second sample or third test if the first is negative.
If he has a tense painful abdomen, continues to vomit even with the acid reducers, becomes very lethargic, or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible on an emergency basis as anything you give him orally will just come back up worsening his dehydration.
Please let me know if you have any further questions.