what did i do to my knee?

Based off the explanation that you gave, sounds as if you are experiencing patellofemoral pain. This is a very common overuse injury in runners. It can range from being completly debilitating to a minor disturbance. Sounds like its more of the latter, which is good.

It is caused by overuse and muscular imbalances, you do the same repetitive motions over and over. Common causes are weak hips (glute medius) and sometimes excessive pronation as well. There are other contributing factors but I wont go that in depth.

I would recommend adopting a good warm routine before your workout stretching and icing afterwards as well as including some basic strengthening exercises like mini band walks and glute strengthening. If you know someone who knows how to do McConnel taping that can be useful as well.  

Agreed...sounds like PFPS aka "runner's knee"...if it's just to the inside of the kneecap, that's typically what it is.  If it flares up after you've been sitting down or hurts going up or down stairs, that's pretty much the classic indicators.  If you aren't experiencing particular pain in those situations, it might be something else.

If it is runner's knee, you want to try to avoid sitting for any length of time with the knee flexed (knee in flexion is apparently when the kneecap gets out of its track at the end of the femur)...find something to rest your foot on to keep the knee straight.  Ibuprofen and ice as with any soft tissue inflammation.  There are isometric exercises meant to strenghten the lower inside muscle of the quads (which is thought to be the usual culprit for the imbalance that leads to maltracking of the kneecap).

I had a bout of it last year...reduced (but didn't stop) training and was diligent about the rehab above.  It was gone within 8 weeks and I did two races (an 18-miler and a marathon) successfully during that period.  You might try a knee strap (the kind for runner's knee or patellar tendonitis)...some people find that provides relief...I did.