For the first time since 2018 May 25 I was charged an additional $10 as a "Flex Rate Data Charge". My plan includes 2GB of data and according to the matching dates on the same bill for data, I used 2.07GB. Does .07GB of data actually equate to the additional charge of $10? For the last 20 of those 31 days for the dates listed for the flex rate data charge, I had transferred over to a new Android phone (from an iPhone) so I'm wondering if I need to change some kind of setting in order to prevent this from happening again--or if this additional charge had nothing to do with getting a new phone. 
Hello Emily9,
Welcome to the community!
The Flex Rate Data charge you note is the charge for data overage usage. The overage rate depends on the the plan. I believe the rate for current in-market plans is $10 per 100Mb. Some older plans' rate might be different. Since the rate is per 100Mb, any overage upto 100Mb would be $10. If you went above 100Mb, it would be an additional $10 for overage upto 200Mb, etc.
In your case, the excess 0.07Gb (equal to 70Mb) used would be covered by one additional 100Mb increment.
I don't think the additional usage is a direct consequence of using a new phone.
Hope this helps 😀
Cheers
