My data plan drasticly throtles down the download speed for the day once i go past my dailly quota, or for the rest of the month if i go over the monthly quota, and they send a SMS notifying me when i go over the quota; the thing is within a couple of days after reseting the counter it already says i've already used more than 300MB which is the monthly quota, but no SMS from the carrier and the speed seems unchanged.

I don't think i've used up my monthly quota this fast; any idea what is going on?