most likely possibilities
patellar tenditinitis...which is an inflammation of the tendon under the knee cap
ACL strain or injury...the ligament
degeneration of the cartilage under the knee cap.
Treatment should right now be ice, ACE bandage, elevation and rest, but you should have an assessment....the best imaging would be Mri if this does not resolve....i would simply have an x ray now, an orthopedic exam and physical therapy
The ligament and tendon problems should resolve with a brief course of therapy, and these are most likely
Let me know if you have further questions
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