Possibilities include nerve impingement/ compression, neuropathy, referred pain from other regions, vascular issues, chondromalacia patellae,
loose bodies, infections/ inflammations, arthritis, bursitis, tendonitis, cyst formation, iliotibial band syndrome, injuries to the internal knee ligaments/ menisci, patellar dislocation, bone growths/ tumours etc; just to name a few. I would suggest consulting a primary care physician or an orthopedician for an evaluation.