Knee stiffness two weeks after impact injury

About two weeks ago, I was standing and holding my laptop and the laptop slipped out of my hands. I panicked and caught it before it hit the floor, but I caught it against the outside of my left knee very hard. The pain was pretty bad and I had to sit down immediately after; however, after a few minutes the pain faded and i was able to walk on the knee with little pain. For the next few days, it hurt to climb the stairs and it hurt a little to walk. A bruise formed above and to the left side of the kneecap but it doesn't really hurt to touch. Now, about two weeks later, I can climb the stairs without pain. There is still a little bit of pain randomly but I can walk mostly pain-free. I have been exercising by using the elliptical, but every time after I exercise the knee is very stiff. Is this normal? could there be a more serious problem than simple bruising? Thanks for any answers.