Inflammation. Knee pain is most commonly caused by inflammation from bursitis, tendonitis, or cartilage degeneration. A good physical exam and possible imaging could be used to confirm the diagnosis. Less common causes of knee pain are irritated nerve roots from the lumbar spine, infection, or tumors. Treatment options will depend on the final diagnosis.
Many things. Osteoarthritis, degenerative joint disease, muscle sprain/strain, fracture, tendonitis, septic arthritis, gout, rheumatoid arthritis, reiter syndrome, acl tear, PCL tear, medial meniscus injury, lateral meniscus injury, and osteosarcoma to name a few. See a doctor if it bothers you for more then a couple days.
Diagnosis. There are numerous reasons for knee pain and an evaluation is generally required. Most orthopedics exams are directed at identifying surgical problems. Patients are often surprised much of knee pain is poor control at the hip and ankle. Two YouTube videos review the concepts: https://www.youtube.com/watch?v=43Nh5aYwFnA https://www.youtube.com/watch?v=t6SXiDCMP3E.
Multiple causes. Knee pain can have multiple causes including but not limited to trauma, arthritis, infection, and neoplasm. A thorough history and physical followed by diagnostic imaging can help get the answers and treatment you might need. There are many non operative treatments for knee pain once the cause is determined.