I hope it's not cancer but let me tell you about Bailey anyway just in case. Bailey was off her food and picky for about 6-9 months before her diagnosis (splenic fibrosarcoma). What made me take her to the vet though was I felt a hard mass in her tummy while she was roaching. I compared with Murphy's belly and his was soft. The vet felt the mass too and confirmed it with abdominal xrays. So, palpating for a mass is important and maybe some xrays or ultrasound.
Good luck.