Lower back pain complaint is common in patients who have had a hip replacement surgery. This is mostly associated with changes in body mechanics caused by osteoarthritis.
Also, inflammation of the tendons around the hip muscles can irritate the surrounding nerves and cause pain in the groin region including the thigh and buttock.
The pain may also be associated with the change in leg length for which you may have to undertake leg length discrepancy exercises after surgery.
Ideally, back pains should go away after 4 weeks. This may be longer however in persons who have some other health condition such as diabetes or high blood pressure.