Hey y'all

I'm quite new to iPhones and data plans. I'm with fido and got my phone 3 days ago. I have 2gb data and unlimited talk and text. I use wireless and if I use my data plan I turn it off after like 10 minutes. For the free days it looks like I have sent 510mb and received 5.8 gb. What the hell? I have download a couple of songs from iTunes (maybe 10 or something) but how the hell is that equal to 5gb???? I read some stories how people ended up with $20000 debt because of data. Please help