When bending the knee , i have a litte pain at the back of the knee cap, what is that?

Possibly arthritis: Knee cap pain conditions are common: either patellofemoral syndrome or chondromalacia patella. Both manifest with pain deep or behind the knee cap worse with stairs, inclines/declines, kneeling/squatting, sitting for long periods of time. Anytime the knee is bent deeply, the kneecap engages in its groove on the end of the thigh bone which can cause the irritation/pain.