Look carefully for petechial rash, lymphadenopathy and hepatosplenomegaly which may suggest malignancy such as Leukaemia.
Restricted hip rotation in an otherwise well school aged child may be Perthes Disease.
Failure to thrive, night sweats or a history of a TB contact could suggest TB.
Joint swelling or restricted joint movement of other joints may suggest Juvenile Idiopathic Arthritis (isolated involvement of a hip joint is unusual).
In a well child, with normal examination and investigations, Transient Synovitis of the Hip is likely especially if there was a preceding history of an upper respiratory viral illness.
Fever and an unwell child are consistent with infection (septic arthritis, TB arthritis, acute rheumatic fever, or osteomyelitis) but may also occur in malignancy.