iPad 2 data usage on past billing periods

I have an iPad 2 and I understand how I can see what my data usage has been during the current billing period.  However, I'd like to get a sense for my data usage during past billig periods.  On the iPad 2, settings, cellular data, I can view account and get my current period usage.  If I try to check prior periods by looking at the complete account, it just says the data plan for that month expired - nothing on how much I actually used.  I can't seem to find a way to even view my iPad 2 account on the AT&T web page.  Anybody know?