Woke up with pain in knee, no prior problem

About 3 days ago, I awoke with pain in my left knee. There is no pain to the touch or when I don't move the joint. It only hurts when I extend or contract my knee. Also, it seems that it only hurts when It is extended at a certain angle; i.e. angle of my hip or tibia/fibula, not how far my knee is extended. I can feel a sort of 'pop' sometimes, and the pain seems to lessen as the day goes on. However, it has come back each morning. (despite sleeping in different positions).

The pain is localized in the left side of my knee, below the joint (towards my calf, not thigh). I find it very peculiar because I have not engaged in any athletic or strenous activity, nothing more than walking. I'm a 23 year old male and used to run marathons, lift weights, etc. and never had any knee problems. Haven't been doing any strenuous physical activity for a couple months, at least.

The night before the pain started I went out drinking, but didn't do anything out of the ordinary. I am puzzled, and looking for any insight. Thanks!