10Gbs in 25 days then my usage escalates

My data package (10Gig) lasts till about 23rd to 25th of the month after which my usage escalates to about 2 to 3 Gig every two to three days till the end of the month. I don't understand, I was working fine on 3Gig/Month till the beginning of the last year. Something is wrong with the throughput measurement. Please fix, you are becoming unaffordable! I have called the call-centre and been informed nothing is wrong, how do I prove differently as they do the measurement.