I had knee surgery 12 years ago, i did a terrible job rehabbing it. over the past year i have noticed that i cannot stand on my tip toes on that leg ?

Varies: There are a variety of reasons a person with a prior knee injury might have difficulty standing on their toes on that side. One is if the knee has a small flexion contracture, another is if you have some weakness/ atrophy in your calf muscle. If it is causing you a functional problem, consider being seen and a course of therapy may be beneficial.