I have a pain in the middle of my left knee especially when I kneel on it. But it's not stiff or swollen or even hot only some pain. What it could be?

Central Knee Pain. One thing comes to mind: bursitis. There are sacs over the patella which can get irritated for all sorts of reasons. Sometimes they swell and sometimes not. If you are not having any locking, catching , giving out i would try some over the counter anti-inflammatory and ice the knee.