Compared to other rooms in the house the chances of getting a shock are the highest when you are in the shower, since the water is a conductor of electricity and the resistance of the body is low. If a person experiences a shock in the bathroom , it may be because a circuit is completed when standing in the water and touching another conducting object.
Somewhere there a electrical fault in the system and the current is bleeding into the water supply. The fault could be anywhere in the electrical system and wiring. One of the reasons could be a frayed electrical wire is touching the pipes, drains or the tub. Another reason could be wiring from a plug on the other side of the wall to the bath tub. Some times there may be a short circuit with a wire and the plumbing in a wall somewhere. Also, the refrigerator compressor may be shorting to the ground and the problem gets aggravated over a period of time.
To troubleshoot the problem
Switch off the mains supply in your house and check if the problem still persists.
If you stop getting a shock, call an electrician to fix the wiring. The electrician should be a trained licensed electrician with sufficient experience in both the electrical and plumbing system.
If you continue getting an electric shock, there could be a problem in the water supply or building wiring, so contact the building maintenance authorities and ensure that they repair the problem.
Till the problem is fixed, avoid using the bath tub. Do not ignore the problem, it could lead to electrocution, which is fatal.