Why is there a hot white wire in my ceiling light
Neutrals are hot anytime a load is connected.  That's why we call them "conductors" and insulate them (note this is different than the practice in automobiles). What's supposed to keep them near earth is a solid connection back to the neutral busbar that has much more conductivity than you.   When you interrupt that connection, the load will "pull up" the neutral to line voltage, and will place you in series with the load. 

The sparky didn't see it, because at the time, the load was unplugged/off.  

"But I turned off the breaker!"  That particular load was on a different hot circuit, but was stealing neutral from your circuit.   This is either a wiring error/laziness... Or a multi-wire branch circuit, whose entire design is to do that.  However all legs of an MWBC are supposed to travel in the same cable.