Knee pain esp. while lifting heavy objects, climbing up or down the stairs or rising from a low position like from sitting on a floor may be a sign of early damage to knee cap cartilage. This may be due to chronic deficiency of Vitamin D3 in the body due to lack of adequate sum light exposure in absence of any obvious trauma to the knee. Get your vitamin D3 blood levels checked. Also week muscles of front of the thigh (Quadriceps) can very frequently cause such a pain.

There r many reason for pain.. ligament sprains or tears, muscles strains or tears, inflamation or others.. once u should consult with ortho dr for perfect diagnosis... Thanks... Tc