What are the common causes of pain in my knee?

Many, see list. Common causes of knee pain: 1-arthritis(osteo, inflammatory) 2-meniscal tear 3-chondromalacia 4-patellofemoral pain 5-chondromalacia patella 6-patellofemoral arthritis 7-patellar tendinitis 8-prepatellar bursitis 9-iliotibial band friction syndrome 10-quadriceps tendinopathy.
Many possibilities. Knee pain can be due to many things such as gout, infection, inflammation of the bursa, or due to tendon, cartilage, or bone abnormalities. Have it evaluated by a professional and get the appropriate treatment.