What you describe definitely does sound strange.
regarding the period where you were using 25 GB per day. Do you have any cloud back up services or file sharing programs? The best way to check if this is being generated by your devices is to leave your modem turned off (when convenient) for at least 36 hours and check the usage meter.
The usage meter can be up to 4 hours behind so keep that in mind when checking the usage.
If you still have concerns regarding the usage meter, please contact Tech Support on 1800 834 273 so that they can assist you further.
- Shelly