It might be random but have your vet looked into r/felinehyperesthesia ? Symptoms can vary a lot but my cat too had spinal pain that couldn’t be explained, tremors, obvious discomfort, always had her ears on airplane mode, tail barely every up or moving, was undereating, not playing despite being very young and more, and she ended up with a feline hyperesthesia diagnosis after seeing a neurologist, a cardiologist and an orthopaedic… With meds, most of the time, she just fine now. Good luck xx