First of all, you should definitely stop running until you figure this out. Knee injuries with running are very common, given that running is such a high impact form of exercise. As always, the most important step in healing up an injury
is proper rest from the sport or exercise for that is causing the problem.
There are several potential knee injuries from running. Serious injuries, such as ligament tears, happen rarely but are important to recognize. With a serious ligament injury, you will feel a looseness or slipping in the knee joint, or a 'catch' when you bend the knee. If you feel this, you should see a doctor
More common are overuse or repetitive strain injuries, such as iliotibial band syndrome or patello femoral pain syndrome. These syndromes cause pain and weakness on the outer edge and front of the knee, respectively, and they usually respond to rest and backing off on your training regimen a bit.
The first step here is to see your doctor. They will be able to examine the knee for serious injury and make a preliminary diagnosis, as well as prescribe treatment, which may include rest, anti inflammatory medications, and physical therapy.