Did you have an order which was closed/stopped out without reaching your Stop Loss?

To view the possible reason for this, follow the below guide:

MetaTrader charts are generated using the Bid price.

The closing of a buy order is triggered by the bid price, so you will notice the chart line hitting the Stop Loss.

However, the closing of a sell order is triggered by the Ask price (You would need to enable it on the Properties section when you right-click on the chart).

If you did not notice your Stop Loss hit, you may need to enable the Ask line.