My knee filled with fluid about a week ago, it was fine when it was straight, and didn't have any pain, but it felt REALLY stiff whenever I bent it in.

This happened when I woke up on a monday morning, and I played A LOT of basketball (like 3-4 hours) the day before, but I didn't notice any injuries or anything, just had dead legs after, as always.

Should I be concerned? It's gone down a lot, but I can still tell that something is wrong.

I can mainly notice it on the outside of my knee (left knee)

had this happen to a couple guys at boot. they waited to long to go to sick hall and it fucked with the tendons in their knee's. (they wont be running 12:30 2 Miles for a while now)
as always get an experts advice on this. its very doubtful that JSP as a Licensed doctor that trolls H&F.