Am I having a "senior moment" or is there a phone app available (I have an AT&T Pixi+) that will log your data usage?

I've tried searching the App Catalog various ways, but cannot locate anything.

I have a 200Mg plan that supposedly doesn't count data via WiFi, and I try to limit reading email/Facebook and such to when I'm home, but AT&T is constantly saying that I'm over using each month.

Appreciate any help, or info. Thanks.