In your case, the problem is that there wasn't enough liquidity to execute your stop loss. Thus, your order was closed gradually, as counter orders placed by other traders became available. You can avoid this if you set a stop-market rather than a stop-limit.
Stop limit orders never get executed away from the set price. They only get executed at the set price. Worst case scenario if the market was moving so fast and the stop limit order wasn't triggered in time, no execution would have happened and OP would have been left holding a losing position until the price came back up or even get the position liquidated. For OP's case, I am guessing he used the stop order(market) and due to slippage caused by the fast moving market, his position got closed away from the price he wanted thus that huge loss.