Sometimes using a STOP order, it can execute at the price the market opens at.
If the market opens and gaps down, then the order gets executed but at the current available price.
You buy a stock for £10, and setup a STOP at £8 to limit your downside risk.
The market can open lower than the STOP price though, at say £6 for example. The STOP gets triggered but sells at the current available price, which is now £6 in this example, resulting in a loss bigger than expected.
STOP orders trigger at the available price, so if the market gaps down below your price, it has no other option than sell at this new price, albeit lower than expected.
This is what I think has occurred in your situation.