Hi! First of all, do not panick! You've already made it through the hardest part.
Second, your interview letter is most likely going to come in on February 28th or March 1st. We had to wait for more than a month to get it too. (DQ on 10/27, IL on 11/30).
When I researched the scheduling patterns, I came to conclusion that they only send letters once a month at the end of the month.
So just sit tight and it's coming soon.