Unexplained Data Usage Spikes

I had this happen to me in January.  It's just my son in  I at home, and we work different schedules, so it's rare that we are both home at the same time.  The most I use it for is social network and gaming on my phone.  My son will stream YouTube a couple of hrs a night after he gets home from work.  But he works second shift, so not hours and hours.  We have one day that 300 gb was used.  In the January billing cycle it says we used over 1300 gb for the month.  We usually average 300-400.  All they would tell me is that their equipment is not wrong.