Hey OP, I have had the same experience over the last couple of years. It started in 2010 when suddenly, just like you said, I felt this enormous energy along my spine (felt like being zapped by 1 mil volts). Some mental pictures were associated with this experience. I was quite shocked for some days... Over the last two years I would guess it happened about 3-5 times to me (although in the most recent experience I felt as though I had more control).

The kundalini explanation makes sense to me. It started after all, after I became interested in spiritual practices.

Do not be afraid by it, rather try to learn and practice spiritual techniques. Ask yourself, has this come "out of the blue" or was there perhaps some associated event before this experience?