Replying to Mystery iPhone data usage?

Problem: I have an iPhone 4 and a 500mb/month data limit. I never download apps or stream videos and I basically use my data to check weather/news/kijiji/gmail (never download big messages). IMO all of that should be pretty light on my data yet I'm at or above 500mb constantly. I see that there are apps to monitor how much data you are using but I am trying to find out what program specially is eating all my MBs. Any suggestions on how to find out? Anyone else getting there MB robbed?