As karbin has explained, this can happen if the bid ask spread (bid-ask) widens at the broker.It can also happen if the broker is shady and they are fabricating their price feeds to stop the client.
Concept:
Bid price is price where you sell and ask is where you buy. most brokers show chart with bid price.
Example:
Eur/USD Spread 0.6 pips
bid-1.11531 | ask-1.11537
You sold 1 mini lot at 1.11531 your current loss is 0.6 USD. Stop is say 5 pips
Eur/USD Spread 5 pips
bid-1.11531 | ask-1.11581
Spread widens 1 minute later due to bad broker or news.you are immediately stopped out even though bid price is still the same.
This can happen during news time or during late hours when there is lack of liquidity the spread can widen to lets say 2-5 pips. So this could lead to stop out even though price is not at that level.
There is another situation when spread can widen if the instrument you are trading doesn't have much trading volume lets say a contract of wheat bitcoin etc. Spreads on those things could be huge.
Additionally as karbin has pointed out, only a bad scam broker would widen spread to 15-20 pips during non news hours.So you have ensure you trade with a regulated broker not a scam broker trying to grab your money.
You should post a screenshot of the order and tell the broker name to let us figure out the exact reason.otherwise we are just speculating on possible causes.