Well, without an evaluation it would be difficult to identify the cause of knee pain in your case. Aside aggravation of the arthritis, other possibilities include chondromalacia, tendonitis, bursitis, fractures, patellar dislocation, injuries to the internal knee ligaments/ menisci, infections/ inflammations, arthritis, cyst formation, bone growths/ tumours, nerve injuries etc; just to name a few. The swelling could be due to knee effusion or due to bursitis. I would suggest consulting an orthopedician for an evaluation and appropriate management. Meanwhile you may try resting the knee with some OTC NSAIDS to relieve the pain.
Hope this helps.