I've had an xray and MRI done on my lower back/buttock region for sharp pain. Wen I play sport the sharp pain comes back! Can someone help?

Need exam. It's hard to determine whats wrong. In many back pain cases the xrays can be totally normal yet you may still have terrible pain. If the problem is not resolviing, you need to get an orhopedic exam with a possible MRI to get a specific diagnosis and then treat the problem with the appropriate measure.
Depends. If radiographic evaluations have been performed, then some form of diagnosis was probably given to explain the pain. Based upon that, treatment recommendations specific to your issue should help. If these haven't happened, talk with your doctor or get another opinion.