Impossible data increase - 75% to 100% in 3 days

Xfinity claims that our data usage went from 75% to 100% between Wednesday and today (Friday), 3 days total.  First off, that is impossible for us.  We don't download games, we don't download movies, we haven't been streaming anything the past week and my wife has been out of town the past 5 days.  If anything, our usage should be less than it has been the past 2 months for these reasons.  This isn't the first time this has happened; it also happened during the billing period in November.  That was impossible as well since we were out of town for a chunk of the month.  I'd really love to hear the explanation for this.