What Causes Sharp Pain In The Shin Area While Standing Post Knee Replacement?

When I stand up from a sitting position I am sometimes, but more frequently, getting a sharp pain in the front middle of my leg at just about the panty line. If I sit back down and get up more slowly concentrating on placing my weight on my legs the pain goes away. Occasionally I feel a dull pain in that location which is not alleviated by re-standing, only by resting my leg. I exercise on a stationary bike and pilates reformer about 3 times per week and have not experienced this pain while exercising. I am 67 years old. Could this be arthritis? Also, I had a knee replacement on the same leg in January-doing very well, but I know that leg is about an inch and a half longer than my other leg. Could that somehow be contributing to this?