Hello there,,,,The first thing you need to do after fall from stairs and pain is consult an orthopadecian/musculoskeletal physiotherapist....Xray needs to be taken immediately of the lumbar spine and pelvis to rule out any fracture or SI joint strain or coccyz swelling...only on a proper detail assessment will the exact cause be known and a proper diagnosis can be made.
Next Steps
consult an Orthopadecian with Xray reports sometimes MRI is also recommended
Health Tips
bed rest till a diagnosis is achieved,,,for the timebeing dont do movements which are painful.
resort back to me if you have furthur queries
Regards & Take care.