Hello, I am sorry to hear that Luci is not eating well for you.
There are so many things that can interfere with appetite without necessarily showing any other external signs this may be difficult to pin down, especially early on in a disease process.
If she is a bit of an older cat such things as internal organ failure, chronic pancreatitis or even a mass in the abdomen are possible causes.
If she goes outside unsupervised then eating things she should not like wildlife or toxins are possible causes. Even indoors cats will find things to eat as well, such as rubber bands or toy pieces.
Early on in an infectious process she may not have a fever but she may feel achey and nauseous and as such not want to eat.
If her gum and tongue color are a nice bubblegum pink, she is drinking and keeping water down with a soft comfortable abdomen that isn't painful with gentle pressure then I am less concerned.
You can try some things at home to see if you can't get her feeling well again. If she goes outdoors you do need to keep her indoors where you can keep a close eye on her. The last thing we want is her to slip away and hide, as cats often do when they don't feel well, and then be too sick to find her way home.
At home you can give either:
1) Pepcid ac (famotidine) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and should help settle her stomach and help her feel better and more likely to eat.
I also recommend feeding soft foods mixed with water to make eating and swallowing more comfortable. If she is refusing to eat even soft foods you can try offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If she continues to not eat she should see her veterinarian for an examination, intravenous fluids and supportive care. They can also give her an appetite stimulant such as Mirtazapine or Cyproheptadine.
But if her gum/tongue color are white or yellow, her belly becomes tense or painful with pressure and this has gone on for 3 days or more then it is time to have a veterinary exam. I would not attempt home care at that point as cats that don't eat for more than a few days can get into trouble with severe liver disease (hepatic lipidosis) as they break down fats for energy to live.
Please let me know if you have any further questions.