I am sorry to hear that Violet is vomiting/regurgitating everything she is eating right after she eats for the past few days.
It is important to differentiate vomiting from regurgitation.
Vomiting includes retching and active abdominal contractions.
Regurgitation is often passive, the ingesta just flows out with no abdominal contraction or real effort.
If this is vomiting this sort of vomiting can quickly lead to dehydration, especially if she vomits water too.
If even water is making her vomit you need to take it away from her for now.
In most cases vomiting is triggered by eating something they should not, too much table food, too many treats or something they find outdoors.
More serious causes of vomiting include toxin ingestions, viral or bacterial infections, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, pancreatitis, internal organ failure (kidney or liver disease), or a full or partial gastrointestinal obstruction. Since she is a middle aged pup then an abdominal mass is also a possible cause.
In a young dog, especially if she never finished her vaccine series, a viral infection such as Parvo virus, toxin ingestion, or a foreign body leading to a partial or full gastrointestinal obstruction would be the most likely cause of her vomiting.
If she is vomiting even water ideally she would see a veterinarian now, especially because this has persisted for several days.
If that isn't possible for whatever reason there are some things we can try at home, but if she's not improving quickly she should see a veterinarian for an examination, some diagnostic testing, intravenous fluids and injectable medication to settle her stomach.
To try and settle her stomach at home you can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours
These are acid reducers and may help settle her stomach and get her feeling better. They are quite safe and can be used for several days if necessary.
I would pick up all food for now and water for a couple hours to allow her stomach to settle after the acid reducers.
In a couple hours when you give her water make sure it is in small amounts only. If she drinks too much too quickly that can lead to vomiting. You can also offer ice cubes to lick. To get some electrolytes in you can offer a 50:50 mix of pedialyte and water.
If there is no vomiting for 12 hours offer a bland diet of 1/3 boiled, minced, white, skinless chicken or boiled, lean hamburger and 2/3 boiled, plain, white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow and get more fluids in.
If things go well and she doesn't vomit feed her the bland diet for 2 to 3 days then slowly start to mix back in her regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert her back to her regular diet.
If she continues to vomit even with the acid reducers, runs a fever (more than 103F rectally), or has a lower then normal temperature (less then 99F), has a tense painful belly or if she refuses to eat she should see a veterinarian for an examination, diagnostics, injectable anti-nausea drugs intravenous fluids and supportive care.
If this isn't active vomiting (retching with abdominal contractions) but rather the food seems to passively flow back out with not much effort (which is regurgitation) then we will look at this differently.
How long after eating does the food come back?
Does this seem to happen more after exercise or can it be any time?
Are her food and water bowls elevated or flat on the ground?
If you think this is more regurgitation you might try elevating her food and water bowls and making her stay in an upright position (front paws up on a chair back or table) after eating and drinking to allow gravity to help pull the food and liquids down into her stomach.
Some Boxers have redundant folds in their esophagus as well as skin folds. This allows food and mucous to get trapped and can cause it to be vomited or regurgitated later.
Boxers can also have a herniation of the stomach up into the esophagus which blocks food from getting through properly. This is called a hiatal hernia. They can be born with varying degrees of being affected and may show no signs of it until later in life as the stomach herniates more frequently or to a greater degree. This is called a hiatal hernia.
If she seems to be regurgitating (passive outflow) rather than vomiting (active retching with abdominal contractions) then this can be an early sign of a condition called megaesophagus. Megaesophagus is a condition where the esophagus is dilated and has weak muscles. Food may sit in it and be passively regurgitated back up, sometimes several hours after eating. We will often see food encased in mucous. The mucous is esophageal mucous collected around the food when the food sits in her esophagus for a period of time.
Megaesophagus can be secondary to nerve damage from pulling on the collar/leash too much as well as nerve damage from a foreign body in the esophagus, it can also be due to a low functioning adrenal gland (Addison's disease), hypothyroidism or a condition called Dysautonomia. Sometimes it can be caused by a mass in the chest affecting the nerves that control esophageal function.
Your veterinarian can do a test called a barium swallow to see how his esophagus works. She'll be given barium orally and then a series of radiographs can be taken to see how it passes through her esophagus and stomach.
And they will likely want to check some blood tests as well. I would want to check a biochemistry profile and thyroid profile, looking for things we can treat. Here is a link to a very good article about megaesophagus: http://www.marvistavet.com/html/body_megaesophagus.html
Here is a link to read about hiatal hernias if you would like: https://www.vetinfo.com/hiatal-hernia-in-dogs.html
For now you can try using acid reducing medications to see if they help by decreasing esophageal burn and reflux.
You can use either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tab per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tab per 40 to 80 pounds of body weight every 24 hours.
These acid reducers and should help settle her stomach and decrease the reflux if this is related to gastroesophageal reflux. They are very safe and can be used as needed with him.
I would also recommend feeding her in an upright position and having her stay in an upright position after eating or drinking for at least 15 minutes or so after eating and drinking.
Please let me know if you have any further questions.