There's no way to know what you've done to your knee without a lot more info.  It could be simply strained, it could be a small tear of a ligament, tendon, or cartiledge.  It could possibly be a little arthritis setting in.  If it won't heal or if you run again and it returns, you should see a doctor about it.  My only qualification is that I've had cartiledge issues with both my knees.  Basically the cartiledge loses some of its elasticity as we get older.  I had an old injury from either football or skiing when I was a teenageer.  I lived with it until I was about 35 and was fully functional, but when the elasticity started to go, those little tears in the cartiledge started to get worse and I've had two arthoscopes and can expect more.  Anyway, regardless of what you've done, you should rest it as much as possible to let it heal, ice it after activity, and warm up a little more carefully.  You can find some rehab exercises that will strengthen the muscles around the knee and keep it as stable as possible.  You should also try some anti-inflammatory drugs like ibuprofen, naproxen, and celebrex.  Not all at once, of course.  Each one has some pluses and minuses and seem to affect everyone differently.  I hope it gets better on its own.  Good luck!