eating but underweight

Question
my cat wants to and does eat all day long, i buy her good cat food as well as i feed her boiled chicken.  she has great energy, plays a lot and is alert.  she is underweight though, i can feel her spine and pelvis, and it seems no matter how much or what i feed her she is still underweight.  she doesnt generally vomit and her poop is normal looking.  she's about 10, and she's always been small.  this weight loss has been gradual.  

Answer
I would take her to the vet for blood work  it could just be something like an overactive thyroid and since she's 10 she needs the checkup

also you can feed her kitten food which has extra calories
make sure you change her food gradually

thanks