What your describing has nothing to do wtih your hip flexors. You would not experience this type of pain due to weak hip flexor muscles or even a hip flexor strain. More improtantly how did you injury your knee? Did you feel or hear a pop? was there immediate swelling or bruising? Were you able to bear weight on the leg and walk normally after the injury? Does the knee ever feel as if it's going to give out? Does your knee ever feel like it locks or gets stuck especially after deep knee bend (squat) as if to stand back up? Do you feel any sort of grinding, clicking or popping within the knee, if so what causes this to happen? Does it hurt more to sit for prolonged periods of time? Can you run down a hill with out the knee feeling unstable? Can you make cutting moves without the knee feeling unstable? Can you piviot on the leg without pain? Does it hurt the knee to jump, especially after you jump while in the air? These are the things I need to know. BTW I'm a physical Therapist.
2012 NS Legacy 163.
2013 NS Brothers in Arms Cobra 163.