Your desciption is not much to go on - there are a number of things that it could be, from patellar tendinitis, to patellofemoral syndrome (runner's knee), to internal derangements, including degenerative arthritis. old ligament injuries, or meniscus problems.
As a general approach to start with, try a good over the counter arch support (I like Superfeet), ice, ibuprofen (if you can take that sort of thing), some quadriceps strengthening work (knee extensions), and a good stretching program (quads, hams, calves).
Make sure that your right side is not on the high side of canted roads when you run. Be careful with the amount of downhill running that you expose yourself to.
If this makes no difference after 2 months, or if you have already tried most of these things, see a good sportsmedicine oriented orthopedic surgeon.
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