Hi Renoir111

As the pain with tenderness comes and goes and started suddenly my guess is that you have an internal derangement problem of the knee and that it is most likely something to do with the medial Wikipedia reference-linkmeniscus. Did it start with the truck driving? What were you doing at the time when it first started?

The truck driving is a relatively trivial stress on the knee but with the knee in a bent position and where rotation of the knee joint can occur and the meniscus is in a vulnerable position. It may be that you have some degenerative changes in the meniscus (cartilage) and the particular angle and movement of the knee was more of a case of the last straw that broke the camels back. A common pattern is that part of the meniscus has torn loose from it's "moorings"

If the knee sometimes locks on you or gives way on you this can add weight to such a possible diagnosis

When you are in the mid years, often with a quite physical history like your own, degenerative changes in the cartilage are not uncommon

However this is all guess work, Renoir111. You really should get it checked out. In your work you want your knees in top condition. A physio can help with the diagnosis but you may be better seeing an orthopaedic surgeon for a definitive diagnosis.

If by chance I am right and the problem is quite minor conservative management of working on building up the knee muscles it can sometimes settle down and some adequate repair can occur. However often the problem doesn't settle down and you need a surgical opinion. Arthroscopic surgery is then the surgery of choice.

As I said this is all speculation and it could be a number of things. Only a taking a full history a proper examination and possibly some imaging will really give you a definitive answer. So get it checked out