My left knee sometimes hurts when I stand up, however, it doesn't always. It may also hurt when I'm walking, setting myself down in a chair (but not while actually sitting), or kneeling down. It tends to do so in episodes. So I will go a long time without doing it, but when I do feel it, it tends to go for a short time. I've had this issue for many years (I'm 23 right now), and had grown so use to it that I can't really detail how common the episodes are (as I would usually ignore the pain when it occurs). Sometimes the pain is minor, but sometimes it makes standing up a nightmare.