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 teh things I need to know. BTW I'm a physical Therapist.
first, it felt like the knee popped on/off with immediate swelling and bruising (had that huge knee brace for almost 1 month). Xray said possible Meniscus but never had an MRI as i am still waiting after 8 months--> no one has called. I can run, play basketball (with knee brace of course), but sometimes, without the brace, it feels like its going to "pop" again when i pivot. I know this could be a mental thing. Also when i do deep squats ( no weight), i get the "cracking" sound; like what you do to your fingers... Lastly, i have yet to run down a steep hill, but i've been playing hard basketball lately without discomfort (again with knee brace that keeps the knee cap centered, if im correct)