It is a bug, it happened to me a lot of times in the past when i was trading with monero/bitcoin.
I lost more than 0.05 bitcoins because of that, i was lucky because bitcoin was not even 1/4 of its actual price.. but it is a lot of money if you think about that.
It is a bug, it always happens when you do a lot of trades in one single day, and there is nothing to be worried about, because you probably didnt lost a lot of money in this.
but yes, maybe you should try to send a support ticket to see if they can refund your money.
Do you use a lot of stop-lost on Bittrex? I use two times and both got taken out even the prices had not reached to the stop-lost yet. The bugs seemed look after Bittrex though.