Thanks for posting your query
It might be fracture
or soft tissue injury
in lower back, finally to be decided by an X-ray.
I advice you to go for an x-ray to detect a fracture because there is protective muscle spasm
after injury. If no fracture in x-ray then it is soft tissue injury.
So you have to visit an orthopaedic surgeon
for examination of lower back & x-ray of that region