Hi There, Yesterday I Was Getting Off A Train, Like

Hi there, yesterday I was getting off a train, like I usually do everyday, and I stepped down off of a step and landed unusually hard on my right foot. Immediately I felt my knee absorb all of the shock, and it has been bothering me ever since! It especially hurts at the top of my knee. I have never had anything like this happen before, and today it hurts to walk on it. Is this unusual? I am only 22!