This scenario usually occurs in a Short trade. In a Short trade, you enter with the Bid price and close with the Ask price. However, candlestick represents Bid price only. When the candlestick (Bid price) moves near to your Stop Loss, the Ask price (which is higher than Bid price) would have hit your Stop Loss level, hence closing your trade.
The other possibility is your account has hit the Stop Out limit. Read “FullertonMarkets Margin Call and Stop Out Policy” here.