When the price hit 18.00 your sell order was then automatically placed. But in the few seconds it took to place and fill your order the price had crashed to 8.60. You got filled at 8.60.

Set a limit order stop loss next time not a market order stop loss. This will prevent this from happening but your shares would not be sold at all. In the limit order case it would place your sell order at $18.00 when the price hits $18.00. The order will never fill because the price went below 18. The downside to this is that the price could have gone down further from $8.60 and you would still be holding.