So i was not crazy after all. The same thing happened to me yesterday (19th June, 2017) night too (close to midnight) although i have 2FA enabled with a verified account. Yes i have 2FA enabled even as i write, unbelievable but it happened. I posted in a group to warn members of my plight so they atleast move their hard earned money to their offline wallets and they laughed at me, and blamed for my loss 🙁

I did some search and realized i was not the only one on that day. Check this out: http://highoncoins.com/cryptocurrency-trading-tips/do-not-use-two-factor-authenticatoin-with-bittrex/#comment-12347

I hope in the future Bittrex enables the possibilities of withdrawal confirmation emails even with 2FA, so at least one would stand a chance against the hacker if once email account is not already compromised. Such is the case in Perfect Money and Coinpayments

So my advice is to please keep your hard earned coins/btc offline esp those you are holding for long term and not trading with.

Thank you

NB:
please do not belittle my comments, call me names, call me a liar or worse and think or say to yourself "this will never happen to me". Ask me last week and i would have sung the same song. This hacking business is real and it could happen to you.