If you never had a fracture, you're a bit young to have arthrosis / arthritis. If you don't have fever and the knee is not red, an infection is not likely. What remains is number of laesions, such as your meniscus, your crossbands, retro-patellair chondropathia, etc.
I would go to the dr. Do a fysical examination, perhaps an X-ray. MRI only if he suspects something specific.
