thanks for writing to HCM,
I would suggest you to start doing some basic exercises which you must have done post-operatively, which includes strengthening exercises for quadriceps, hamstrings, gastrosoleus (calf muscles). And mild stretching exercises for the same muscles.
Also it is important to note your body weight, have you gained weight in last 6 months if so then this can also be the cause.
In the most rarest chances your internal fixation
(rod inside you) could have loosen or slightly moved from its original position for which you need to see your orthopaedic surgeon
finally I would advice you to start the exercises under the supervision of your physical therapist and you can also use hot water bag on the painful area.
if this doesn't help, kindly visit your surgeon.
i hope this helps
wish you good health