is very common .
this is mainly because of increase lumbar lordosis
and laxity of abdominal muscles,
nutrional deficiency of vitamin d and calcium which is very common after pregnacy and lactation .
Take proper calcium vitamin d suppliments, abdomonal muscle strengthening excercises as advised by your gynaecologist and the physio therapist .
and if you still have pain after doing all this things consult orthopedic surgeon