This REALLY doesn't sound like a hamstring tear! With the level of pain that you're describing, I would be back at A & E. At the very least I would suggest you find your nearest musculoskeletal/sports physio and get a full examination.

Hamstring tears normally hurt when you stretch them or use them (think hamstring curls), but when the muscle is not on stretch or being used, it is usually painfree (unless very acute with lots of swelling - but you would most definitely have felt the injury occur).

It is possible that it's coming from your back, even if there are symptoms in the hamstring - where is the shooting pain that you describe?

In short, the mechanism of injury and the diagnosis you've been provided do not match up! Get theeself a proper diagnosis.