I am sorry to hear that Pita has a poor appetite, isn't drinking, and seems very lethargic and weak with choking/gagging episodes after being treated with Rimadyl for hip dysplasia pain.
It is possible that the two things are completely unrelated.
Poor appetite can be related to chronic pancreatitis, a dietary allergy or sensitivity, inflammatory bowel disease, heart disease, internal organ failure, a full or partial gastrointestinal obstruction or cancer including infiltrative cancers such as lymphoma or a larger mass in the abdomen compressing the gastrointestinal tract.
Her choking/gagging can be due to reflux or nausea or can be related to heart disease. Boxers as a breed are much more predispoed to heart disease.
But some dogs are very sensitive to Rimadyl and we can see side effects such as gastrointestinal ulcers and upset, organ stress and damage and in some cases failure.
Since there has been such a dramatic change in Pita however I highly recommend that she be rechecked by her veterinarian.
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 today 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 him feeling better and hopefully get her appetite back. They can be given as needed for several days.
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 her a 50:50 mix of pedialyte and water.
If there is no vomiting for 4-6 hours offer a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled, white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow. If she refuses that you can offer a little meat baby food. If she refuses both then don't push it, she needs veterinary care.
But if things go well and she does eat, doesn't vomit and seems to feel stronger then 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 eat poorly or starts vomiting 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 she should see a veterinarian for an examination, diagnostics, injectable anti-nausea drugs intravenous fluids and supportive care.
Please let me know if you have any further questions.