Probably you have traumatic lumbar disc bulge esp at L4L5 and due to which you have sevrre back ache and features of urinary retrnsion.
If the pain severe associated witj bladder symptoms you may need urgent surgery.
Kindly consult a neurosurgeon. In the meanwhile take absoute bed rest
. LUMBO SACRAL CORESET need you need wear. Take pain killer and pregabalins.