Pain in the inside of the knee can be caused by medial meniscus tears, arthritis and medial collateral ligament injuries.

There are two moon shaped cartilage that act as shock absorbers for the knee.  The medial collateral ligament is connected to the medial meniscus. As such, any impact on the lateral side of the knee can cause injury to the meniscus.  An impact example is the twisting of the knee, like when you get tackled in football.

If this happens to you, you should rest, ice, elevate and compress.