Reliance Netconnect Broadband+ — Wrong Internet Billing

I am a customer Of Reliance Net Connect Broadband+. I have a plan of 1099 Day/Night and with that I have addon of +199. According to that I have 10 GB at night usage. For additional, 2Rs/MB. 5th is the bill generation date for me in every month. This Month My Bill came 39701.53 Rs. But According to the net it was showing my total usage is 16009 MB. So haw come it possible to generate a bill like this?? According to my plan I have 10GB at day and 10GB at night. Total it comes arround 20 GB. They are cheating the customers like this only. I am simple student. If they will cheat like this how come I will survive from this????? Please help me out.