I am sorry to hear you are worried about Lily. I am worried, too. My name is Rebecca. I am a veterinarian and will do my best to help.
Dogs tend to hide their symptoms when they are sick. Sometimes the only way they let us know that something is wrong, is by developing a picky appetite, or not eating.
So we know she is telling us she is sick. Is she vomiting at all? Is she still drinking water? Is she drinking more water than usual?
If you look at her gums, do they look pink? Or are they pale?
If I could, I would start with a good examination: Listen to her heart and lungs, take her temp with a thermometer (normal is 101 to 102F), feel her pulses, palpate her internal organs and lymph nodes.
If she has any symptoms such as vomiting, excessive thrist, coughing, limping, I might be able to help figure out where her problem is. If she is just not eating, she is telling us she does not feel good; it could be anything from pancreatitis, to heart disease, kidney disease, infection, or even an internal tumor. How soon can you get her to a veterinarian to be checked out? Until then, try to keep her hydrated by offering ice cubes, broth, water, or Pedialyte.
Let me know about her gum color, if she is drinking a lot, etc. and how she is doing. I am here for more of your questions.