knee pains-

Hello folks- I am, or consider myself to be a fit male, age 30. My girlfriend and I recently began running on sidewalks instead of the local composite track. My right knee is now killing me. The pain is to the lower left of my right kneecap. To bend my leg, as if I were in a stride is excruciating. I never heard a pop or anything to suggest an injury. Is this a stress fracture or ligamnet damage??? Thank you for any info. Hawker12