I should differentiate between a knee INJURY, involving a blow to the knee and a RUGBURN, which is due to a scrape. Generally, if you fall on a knee or twist an ankle the drill is cold packs and wrap with circumferential ACE bandages immediately, before swelling takes place.
If there is leg swelling and redness from a rugburn there is the liklihood of an infection. These do not resolve on their own. See a physician. You will likely need an oral antibiotic. An x-ray will be meaningless. You have to be careful because there are many nasty new bacteria out there that cause big problems.
Hi clean area and apply an Antiseptic Cream twice a day , knees do take longer to heal , but if you have knocked it and done damage please get it Xrayed ASAP , take care ,
i forgot to add that i also fell very nauseous