Check your history. Probably you didn't enter your username, password and 2fa code on Bittrex site but on a site with a very similar address.
How did you get to their site? Searching Google for their name or address? Writing the website address?
i really hope that this is the culprit. it sucks that the OP lost money, but if the exchange had its TOTP token database compromised, there will be big losses coming for other customers.
i haven't heard any other complaints like this yet today, so i'm guessing he just got fooled into clicking on a fake site / phishing link.