Hi there. The various causes of severe knee pain could be a condition called chondromalacia patellae caused by damage of soft tissue underneath your knee cap. this can cause pain when the pressure is maximal between the patella and femur is the greatest with the load being applied to a bent knee like when climbing up and down stairs, getting up from chair or floor, squat, kneel etc. Therapeutic options include exercising your quadriceps. Other possible causes can be overuse of knees, poor form during physical activity, not warming up or cooling down or inadequate stretching. Being overweight could also cause many knee problems like arthritis including rheumatoid, osteoarthritis and gout. bursitis due to inflammation from repeated pressure on the knee, dislocation of knee cap, tendinitis etc. are other possible causing for the excruciating knee pain worsening on knee movement. Consult an orthopaedician with these contentions. Hope this helps. Take care.