Unreasonable Stop Loss Price Taking

I was trading NZD/CHF with them several days ago. At first, everything was going really great and I was in a profitable position. Because of the nature of my strategy, I don't have to check my charts every so often, so went and do my daily activity as usual. Only checking the positions every once in a while. Suddenly I was out of the market! Oanda kicked me out of it even though I was in a good place and nowhere near loss. The price was nowhere near my stop loss but they stopped me out. When I questioned it, they claimed it was a market slippage that caused it which is fair enough but what annoyed me was taking £18 instead of £11 the agreed stop-loss price.