My billing is on the 28th. This month on the 29th I got a notice that I have used all my monthly data. How can that be when I have not done anything different from the past? Can your equipment have gone bad or got hit by lightning?
Check your usage now and see if it still says you've used all your monthly allotment. You may have exceeded your usage on the 28th and the notification email was generated and sent to you from that excursion.
It is typical for those usage emails come two or three days after the limit was reached. As wmBama suggested, checking the usage meter is the best way to see how much you have left in the current cycle.