Sheryl, You must slow down,the problem is only with the bathroom light,correct?
If that is the case then forget the rest of the house,forget about switchboards.
If you want to look further yourself you could remove the bathroom light switch cover, connect your meter, set to 250 AC and with the light switch in the OFF position, test the 2 wires connected to the switch, the meter should show near full scale deflection,if it does then test again with your heater in action,if the deflection is the same then your heater is not a problem.
If the meter deflection is reduced then you have a supply voltage problem This sould make further diagnosis easier,maybe loose connection at switch or light