If a market trades over your limit level then you will be closed at the best available price at the time. This price will always be at your limit price or better, and is referred to as ‘positive slippage.’
Your stop triggering acts as a request for us to go to market, and close the deal at the best available price. This stop market order can be filled at a price worse than the level you selected, resulting in negative slippage. Most markets have the option to add a guaranteed stop to the trade, which would guarantee the exact level of your stop.