Depending on your age this could be something called Osgood–Schlatter ("growing pains"). I had this in HS, and it was extremely painful to the point that I had trouble going down flights of stairs, and they ended up putting me on a Rx strength painkiller for a while until it subsided. You could get your aerobic training in on an elliptical or stationary bike in the mean time. I actually really the recumbent bikes because you can sit there and read while you pedal, the only problem is that to get one that has a decent design of the magnetic/flywheel mechanism in your home, you will be spending a small fortune. I always felt that running had a pretty brutal effect on my body and I never felt good after doing it, although training to do a "neutral strike" also helped a little bit for me, personally.

It might be worth looking into doing some stretches as mentioned in the wiki article (citation 18 looks like a solid source):

Prevention

Example of how to stretch the quadriceps muscle.[6] One of the main ways to prevent OSD is to check the participant's flexibility in their quadriceps and hamstrings. Lack of flexibility in these muscles can be direct risk indicator for OSD. Muscles can shorten, which can cause pain but this is not permanent.[17] Stretches can help reduce shortening of the muscles. The main stretches for prevention of OSD focus on the hamstrings and quadriceps.[18]