Why does my knee hurt??

I went for a run with my sister on Thursday night, my last night in las vegas. We probably only did less than a mile barefoot and it felt great! I probably shouldn't have done it barefoot, but my shoes were already packed and I really love running barefoot. I also did about a mile in tonopah yesterday, but I did that run in shoes. That felt pretty good too, but I wasn't following ChiRunning form, I was just running in my normal sloppy form. There were lots of hills, loose dirt, and rocks so I was not sure how to include a lean and a mid foot strike on that type of terrain without killing myself.

Today (saturday), I was running some errands up in reno where the weather is about 35 degrees and cloudy, but I love it. I walked up a flight of stairs and the outside of my left knee (IT band) was hurting! I dont understand! My knee was feeling great when I went on both of those runs and I have been stretching it and using the foam roller every day sometimes twice a day. Could it be because of the cold?

I really don't know why it is happening. The only explanations that I can come up with are: Barefoot running too much, my IT band is getting weaker, and the cold weather.

I am going to do a short trail run today in shoes, and hopefully that wont make anything worse!