from the sounds of things it could be a meniscal injury (partcularly if it ocurred whilst twisting the leg) in which case you should probably be visiting a doctor or physiotherapist. Also - if there is any instability in the knee it's worth getting checked out.
If there is swelling I would suggest standard RICE priniciples for the first 48 hours - rest, ice (20 mins every 2 hous), compression and elevation where possible.
Good luck and don't be afraid to visit the doctor - I'm sure you are not the only person to have injured yourself in this way and I can promise it won't be the wierdest injury story s/he has heard.