Airtel Broadband — Billing Variations

I am the Airtel customer for the Broadband for past one year.
From last month I have noted a high raise in the bill which was told that I am downloading at the specific band width which was allowed on call or request. However I did not recall any such incidents that I have requested for any such bandwidth. Since I was travelling I did not have much time to clarify that and I paid and left.
I had also requested to downgrade my plan and opted for the 499 plan as exams were approaching and usage of computer is going to be limited.
However I was shocked to see another enormous bill amounting to 1953. I can promise that we were not in station for many days and the modem was switched off (so there are no possibility of other using through the WIFI), now after we came back even surfing is very minimal as exams are approaching for my daughter.
I have lodged a complaint with the Interaction Id #[protected] and my compliant #30113970.
There is no details provided , like the IP address that the downlaod thats happening , when asked the relevant details are not provided.
I am very sure about the amount of hours usage and downloads that I have done.
I seek justifications in this regard
Anita