Which data counter are you using? The one bundled with the phone? It's pretty hard to tell retrospectively as far as I know.
Your best bet would be to check the counter against what your carrier says, there will most likely always be a little bit of difference, but I've found the stock android data counter relatively reliable.
However, if you're looking for a 3rd Party app, 3G Watchdog and Onavo are both pretty good alternatives