The same happened to Remy... she went from 3.5 to very close to 5
She was spay at about 14 m. also... and the charts said she would be 3.5 so I was a bit surprised. But to be honest.. she was too skinny and was glad she gained some.
With the advice from my yt friends I started adding veggies... green beans and such to her food. I went onto the balance-it web site and purchased a "recipe" for her.. and I mostly feed her home cooked food per recipe and advice.
She has leveled out at 4.5 -- not too bad. She looks good, and is healthy.
I agree with the exercise -- we throw toys and play chase in our very small back yard. We also walk her twice a day... about 20 to 30 mins each.
good luck