Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Milly's retching episodes for the past 6 weeks. I am glad to hear that her exam was relatively normal.
I suspect that they believe she may have some tracheal collapse, which is secondary to weakened rings of cartilage due to age or repeated trauma to her neck from hard pulling on the leash. These pups often worsen with exercise.
If this seems to be bothering her more at night though I am concerned that she is suffering from gastroesophageal reflux. Gastroesophageal reflux makes sense if it is happening more at night when she is laying down because reflux happens more easily when they are laying down, the acid flows more easily from stomach to esophagus when she lays flat.
Gastroesophageal reflux can be a primary disease process where the gastroesophageal sphincter weakens and allows acid to reflux up into the esophagus which burns the esophagus and sphincter, worsening the sphincter's weakness. It can also be secondary to internal organ disease, like kidney disease, or inflammatory bowel disease or even infiltrative cancers. Sometimes an enlarged abdominal organ will put pressure on the stomach causing reflux.
When the acid refluxes up the pup will often gag and swallow repeatedly. It is painful which can cause them to pant too. Gastroesophageal reflux can respond to dietary management and acid reducers but if there is an underlying disease process we do need to address that as well.
It is important to feed frequent small meals of a bland diet (like Hills i/d or Purina Veterinary Diets EN), or a hypoallergenic food if her reflux is related to dietary allergies (such as Hills z/d or Purina Veterinary Diets HA) if this is a long term problem. Short term you can feed a homemade bland diet of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat, and 2/3 boiled white rice.
To decrease stomach acid production and this decrease the burn you can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 9 to 20 kilograms of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 10mg tablet per 9 to 20 kilograms of body weight every 24 hours.
These will reduce stomach acid and should help decrease reflux, or at least decrease the burn. They are very safe medications and can be used long term if need be.
In some cases we do need to use prescription medication to improve gastrointestinal motility and improve the sphincter, like Reglan (metoclopramide).
If she is not improving with the acid reducers and dietary management she should see her veterinarian for a recheck examination and further diagnostics as well as possible prescription medication.
If she is not improving I recommend checking a complete blood count, biochemistry profile and thyroid profile as well as radiographs of her neck and lungs.
Please let me know if you have any further questions.