Out of nowhere today, i'm having problems with my right knee. there's pain right underneath the kneecap deep within. i didnt hurt it, what could it be?

Knee pain: Knee pains usually require an in-person examination to determine the cause. Kneecap pain can come from irritation from the kneecap on top of the other knee bones. This can come from poor alignment of the knee tendons and muscles. This is called patella-femoral syndrome. It is best treated with specific leg muscle stretching and strengthening exercises.