Data usage

This past couple  of month's  have been very frustrating with going over data usage.  We usually come close to the wire the last few day's and use about 1% a day.  But for some reason  within 2 week's  we are at 85% nothing  has changed with our internet  usage.  We got up to 105% so I shut the modem off for a week until it rolled  over into the next month and when I turn it back on We somehow managed to jump from 105% to 111% with absolutely  no internet  use...  How is this possible???