Please note the unexpected usage could be because there are features like Adaptive Wifi or Wifi Assist. Those are new features on you phone to automatically switch to Data if you have a Weak Wifi signal. Check if that was what happened
Also, PM cycle does not start on the first of each month. In fact, the cycle start date keeps changing, because it is using 30 days cycle. So, you could be on a 3rd or 4th week of the current cycle.
For the usage, you need to confirm couple things first
Login to My Account, check the Overview page, there will be a line about Data in My Data & Add-ons, check if you see a line about data like the following, it will tell you how much data you used:
Then you can go to Usage History and then filter from the beginning of the current cycle and look for Web Traffic. You can briefly sum up if the number is correct. (Again, check your current PM cycle start date. Also do not trust the sorting feature there, it looks sorted, but it is not. So, really go through each page):
Now, if you are on Android, you can check against your Mobile data usage and see if it matches. To make it works, you have to change the cycle starts data on your data counter on your phone to match your PM cycle start date on this month
(if you are on iOS, although there is a data usage there, but it cannot easily tracc back if you did not reset the counter on the exact date of the beginning of this cycle, so, it is not very useful to trace back)
