without knowing the full extent of the problem as i have not seen your knee, the swelling you got may have been a reactive synovitis or maybe an infection? either way my advice is to get into physio asap and have your knee assessed and some gentle hands on mobilisations performed and to ensure the swelling is well controlled and continuing to reduce. you are very right in your concerns over the knee becoming unecessarily stiff. a good physio wil be able to do what is right for the state of the knee and encourage movement and strength whilst respecting and controlling the swelling.