Some info regarding knee injuries.
Additionally if you haven't gotten fitted for a pair of running shoes, then that may also be a cause for your pain. If you already own a pair, then it may be time to replace them.
If you haven't been fitted for a running shoe:
Runner's World list of specialty running stores in the U.S. and Canada
There's a lot more to a perfect fit than finding the right size.
I know you don't want to hear this, but I would slow down or stop exercising for a few days in order to rest your legs. And yes it sucks, but running on an injury will likely make it worse and lengthen your recovery time. If it makes you feel any better, I have to take a week or two off from running in order to heal a pulled muscle.
It's also possible that you are running too fast on the treadmill or that you added extra minutes before your body was ready. I would try running at the lowest speed you can for awhile. For example, I can run at 4.6 on the treadmill (14-20 minutes max), but my posture suffers since it is a bit too fast. As a consequence, I injured myself.
However I can more comfortably run at a 4.4 or 4.5 pace without hunching over. I wear a HRM during my workout and the intensity hasn't decreased with the speed. However don't use my speed as a guideline because everyone is different and you may be faster or slower than I am.
FWIW, I started on the treadmill 25 pounds ago and the only times I have been injured was due to the shoes wearing out and pushing myself too hard and overdoing it. Overcoming the Overdoing It Demon has been a struggle for me.