While it is not unusual for dogs not to eat the first 24 hours, either due to nausea, poor gastrointestinal motility or pain, or leftover grogginess from anesthesia most dogs are eating something 48 hours out.
If the discharge is very bloody, or white/yellow in color then I am more concerned.
If her abdomen looks a little swollen, her discharge is bloody and her gums are pale that can signify internal bleeding. She should be examined immediately on an emergency basis.
If she is running a fever, or has a subnormal body temperature that can indicate a secondary infection, and she should be seen on an emergency basis especially if the discharge is cloudy.
If she didn't come home on pain medication this may be related to pain.
If her color is good, she is drinking OK, and she has a little blood-tinged (pink NOT red) discharge from her incision, and she doesn't have a fever she may just be recovering slowly, and getting groggy from pain medication.
It may help to warm her up with hot water bottles, give her an acid reducer to relieve nausea and feed a soft, bland diet.
To try and settle her stomach today you can give 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 from the anesthetic.
Then start a bland diet of 1/3 boiled, lean hamburger (or boiled, white skinless chicken), all fats drained off, mixed with 2/3 boiled, plain white rice. Feed several small meals a day. You might wish to add 1 tablespoon of canned pumpkin to each meal as fiber helps stimulate gastrointestinal motility.
If in doubt about her condition and she is very lethargic the safest thing would be to have her seen, especially if she isn't drinking. Then can give some fluids, give an injection to counteract nausea and make sure all is well.
Please let me know if you have any further questions.