I agree with Farmerman. I see the labs came back normal, but there might be a further test. If in doubt, we always ask for a complete blood workup, but you may already have done that. Sadly, all such tests are rather expensive...
How old is your cat?
We rescue cats and have a large number living with us now. Most are over 10, some are over 15, one is now 20. Our oldest died at 23 or 24.
We recently switched all the cats over to kidney support food, and everyone's energy levels rose rather swiftly. Also, gingivitis and other mouth infections are common in older cats -- ours are checked regularly for mouth infections. We have 2 that have mouth infections and have to be kept separate from the others, as this is highly contagious.
Also, older cats seem to prefer more liquid in their food, like soupy foods or with lots of broth. Not so much made by adding water and mixing, as that which comes from the packet or can already mostly liquid. Especially if their teeth are bad or they have a painful mouth infection, they don't like to chew or crunch dry food and prefer something easier to eat.
Ask you vet about antibiotics, but be careful about the length of time to use them. Antibiotics are hard on the liver and kidneys for older cats, especially if they already have a renal problem.
Please keep us posted. There are a lot of people reading this who have far more expertise to share than I do...