I'm sorry to hear that your pup Rhianna has vomited twice in the morning and seems to have less of an appetite than usual after her spay surgery.
Her vomiting/nausea may not be directly related to the surgery itself, but it may be related to the effects of anesthetic on her gastrointestinal tract.
Anesthesia slows down normal gut motility and while they are under the influences of anesthesia it also relaxes the gastroesophageal sphincter. As such we can see acid reflux which can burn the esophagus and the sphincter itself.
That hurts and so dogs may be reluctant to eat.
An inflamed, swollen sphincter also is not as competent, which leads to more reflux, worsening the problem.
To decrease stomach acid production and burning at home you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These are acid reducers, are very safe, and should help her feel better. They can be used as needed, and I would give them for a couple weeks if they seem to be helping.
You can also try a bland diet with increased fiber to perk her appetite, ease irritation and get her gut moving. A homemade diet for this is 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat, and 2/3 boiled, plain white rice. Add 1 tablespoon of canned pumpkin, (not pie filling, just pumpkin), to each meal for fiber. You can also add low salt beef or chicken broth to her meals as well to improve palatability and get more fluids in which should help as well.
Once she is back to eating well you can slowly convert her back by mixing in her regular food, adding a little more regular and less bland at each meal. It should take a week to get her converted back to regular food.
If you aren't able to get her to eat anything over the next 24 to 36 hours, she begins vomiting consistently or running a fever (rectal temperature greater then 103F), or her incision looks red or swollen or is draining then she should be rechecked by her veterinarian.
Best of luck with Rhianna, please let me know if you have any further questions.