I Have No History Of Any Knee Injuries. About 4

I have no history of any knee injuries. About 4 days ago I started feeling like I had an air bubble under my left knee cap, like it needed to "pop". It's gotten more sore as the days pass. I've been off of it for 3 days but no better. There is no sharp pains, just sore and the bubble feeling. Any idea what it could be? Thanks