Sounds like glitch. But for peace of mind, it’s not a bad idea to disable all background use of data for your apps, allowing only specific apps as you please. You can also keep your cell data turned right off most of the time, to avoid operating system drains. (It can happen, even if the settings are locked down).
I noticed my self-serve account actually changed data use amounts, dropping back several mb (perhaps fixing mms use vs. actual data use.) Keep an eye on your account, though, and maybe note down what’s recorded, just in case it goes crazy and you need to contact pm moderators.
