You might just be experiencing the pain from the impact still, but it's always better to get it checked out rather than wait and let it possibly get worse... I would definitely use ice on the knee in the meantime. Put solid ice (no towel in between the ice and knee) on the knee and swirl it in circles against the knee, keeping contact the entire time. Do this for seven minutes without breaking contact with the ice and skin. You will feel your knee go through a strange cycle- first cold, then pins and needles and eventually numbness. You want your knee to go through this entire cycle and it will only do this if you keep the ice in contact with the knee, but keep the ice moving around it. Be sure to do nice, easy, gentle knee stretches before and after the icing.
If you are still worried, I would see someone who specializes in the joint such as a chiropractor, physical therapist or orthopaedist. I hope your knee feels better.