Hello.. little more detail would be appreciated. As u r just 23, there must be definitely some strain like u went for a trek, or u did suddenly some new or vigorous exercise or activity. It can also happen because of occasional workout like dancing just twice in a week, or any severe activity. It can even happen due to unprepared marathon. As of now please apply ice on the swollen part of the knee n consult a physiotherapist as soon as possible and u will have to take treatment for a week