What counts as data usage on the iPhone?
Everything that is not either a) texting or b) phone calls uses data. (That is to say, everything that sends or receives something.) Streaming music uses a lot of data (streaming video uses even more). Photos only count when you send them or receive them. 

Also, note that the phone will use a WiFi network if available instead of the cell network, so that data doesn't count.

Your bill should give you some idea of how your data usage increased; if it doesn't a call to AT&amp;T customer service (or visiting one of their stores) might be helpful.