why does my knee hurt when i straighten it

The pain in the knee when straightening it can be caused by various factors such as inflammation or injury to tendons like hamstring tendonitis, presence of a Baker's cyst, strain in the popliteus muscle behind the knee, meniscus tears, arthritis, or even a blood clot (deep vein thrombosis). It may also stem from ligament injuries, cartilage wear (chondromalacia patella), patellar or quadriceps tendon issues, or muscle imbalances. Symptoms that often accompany this kind of pain include swelling, stiffness, popping or clicking sounds, restricted motion, or a feeling of instability in the knee. Treatment depends on the cause but typically includes rest, physical therapy, medication for inflammation, and in some cases, surgical intervention if conservative treatments fail.