Sharp, back pain affected by movement

Patient: I hurt my back somehow. yesterday, it began off as a small, stiff, cramp like feeling in the middle of my back and i didn’t take notice, today i woke up and it was significantly worse! I took two paracetamol, and was moving around fine, but then out of the blue, it became unbearable, i went to pick up some washing and ended up feel a sharp contraction. I’m a twenty year old male, who’s relatively fit. what could it be?

Symptoms: Sharp back pain affected by movement of arms and legs, muscle contracts every now and again.

Doctor: