So you brought your own modem?
Things will be clear only after you have got the copy of the bill as it enlists detailed charges break up.
Right now you probably won't even be able to register on the selfcare portal as it asks the 10 digit bsnl account number which is printed on the bill itself. Else you'd have been able to download pdf of the bill from 5th of any month when bills get generated.
Also just curious what was your connection start date.