It sounds like your fall led to an injury to the front part of the knee around the knee cap. This is common when the fall happens on a flexed knee - lots of pain, but it generally gets better quick.
Since the symptoms don't include the tell-tale signs of an internal problem in the knee, like severe swelling and the inability to weight bear, this most likely is just a contusion or bruise.
Bruises that continue to hurt a week or so after an injury can be related to several things. When the injury happens on the tip of the knee cap, though, I always think of a bursitis problem. The bursa is a small fluid sac that lies under the skin and above the knee cap bone. If the knee cap is hit really hard in this area (and doesn't break …), the tissue called the bursa can bleed and swell and hurt. This area can take a lot longer to heal than just a simple bruise, but generally it should go back to normal. On rare occasions it will stay swollen and even look like a golf ball on the knee. Odd looking, but normally not a long term problem.
Time is always a good option for healing a knee injury, especially when the swelling and pain are improving. At any time, though, if the symptoms worsen, I would visit your PCP for an evaluation and a possible x-ray.