Notification from my mobile provider, this afternoon: You have 349MB of mobile data allowance remaining
So, uh... I did some digging, and it turns out that the biggest culprit was the tumblr app. And I was confused, 'cause I've not been using it any more than I was previously.
Turns out, it's a combo between autoplaying video stuff that loads as I scroll (as it always has), and work changed their wireless router, and the new one has a different wifi password so I've been using my mobile data for all my timewasting! DAMMIT!