Off the cuff the diferential would be pes anserinus tendinopathy or bursitis. Overuse running injury like with Iliotibial band friction syndrome (runners knee) except on the medial (inside) aspect of the knee. Treatment would be rest for a few days, heat pre-exercise & ice after and get contributing factors checked out (footwear, biomechanics, etc). In severe cases of bursitis injections can be effective but strong chance of re-currence if the contributing factors not addressed.

Hope that helps