this normal or am I screwed iPhone 3G

Hi, I just got a $25 500mb data plan from my carrier Fido. When I go to Settings>General>Usage and look at my Sent and Received data, I notice that it it keeps increasing. For example, it will be at 605kb sent and 4.6mb received and then when I recheck it 10 minutes later, it will be at 620 etc 4.7mb received. I know the iPhone always uses data even when I am not using it, but is this normal. The iPhone is doing this on its own, in this time from say 605kb to 620kb Ihave not used any data myself. It is all automatic. I mean, wont my data run out before the month is over at the rate this is going?
PC Windows XP