Hard to say, just my guess ..... running can sometimes lead to imbalances or tightness, which can start out without symptoms and then one day you just start getting pain and the symptoms start.
Its probably best, if possible, to see a PT who is experienced in treating runners who develop patella pain. It is not an uncommon scenario.
Based on what you've described, at first what I would probably try is a foam roller over the IT band. I wouldn't be surprised if that is very painful for you to do, at least initially. Also, stretching the IT band might help. Lastly, I would try a heel to butt quads stretch, also known as prone quad stretch, and see if you can get your heel to butt. Possibly also try a little heat over the quads and then massage it gently with a tennis ball and see if you have some painful areas, especially on the outer quad.
If you have a lot of tightness doing the above, then you may have success with PT to strengthen other muscles that don't get as much work during running.