Painfull knee?

about 5 years ago i twisten my knee and had pain on and off for about 4 years. it then went but in the last week,in the same knee i have been feeling a different kind of pain.when im walking it every step or so it (its hard to discribe!) ...sorta clicks but not a click that you can hear but the sort you can feel if u know what i meen. its also become swollen and very stiff to move.im getting pain more on the right side of the knee but also the back.not pain like i cant walk or anything but a uncomfatable pain .i can wlk up and down stairs ok . if i try and cross my legs it stiffens up and cant cross it properly. and alot more painfull if i havent moved it for a bit.what do you think this could be?