Were you withdrawing bitcoin? What fee is bittrex paying for each bitcoin transaction?
This mempool chart shows the number of unconfirmed bitcoin transactions in the network is approaching the highest its ever been.
https://blockchain.info/charts/mempool-size?timespan=all
The bitcoinfees estimator website recommends paying 700 satoshis/byte for fast transactions today. If bittrex is paying lower fees than that withdrawals could takes a very long time.
https://bitcoinfees.earn.com/