Why does my iPhone 4 keep using my data

So yesterday I gave this girl my phone to use and she didn't tell me that she needed to download a PDF on my iPhone, but she did it, and right away I got a notification from AT&T saying that I had used 65% of my data plan. So right away, I went and turned my location services, my cellular data, and my 3G off. They've been off since yesterday morning. Then just now, I got another notification from AT&T saying that I've used 90% of my data plan! And I haven't used any data since I got the 65% data usage, so I'm not sure what's going on? I looked on the apple support page and people have said that it's apps that use the data? But if I turned my 3G and data off, how can it use the data? I'm really confused and very worried because I just went over my data plan last month because I wasn't sent all 3 notifications that they say they'll send (65%, 90%, and 100%; I didn't get the 90%). I can't go over again otherwise my parents were already angry that I went over last month, and I feel so bad! What should I do!

You need to call your carrier and find out what's going on, double tap the home button and make sure all apps are closed so nothing is running in the background. If you want to stop it for sure then turn on airplane mode (which means Wi-Fi only) until you figure out what is going on.
Mine doesn't go up at all when not using so something is weird here… I would also explain to your parents you are doing everything you need and this is happening.

Another lesson here is don't lend your phone to anyone to use, no reason for them to be messing with your phone unless its a quick phone call only.