Why is my data usage so high?

So I recently got an iPhone 4, I've only had it for at least 3 weeks. My mom got a message tuesday saying that I used 65% of my data. (we share a plan, 300mb in data.) the only apps I used were instagram, gifboom, pandora, twitter, matching w friends & temple run. After getting that message, I deleted all of the apps except temple run, pandora, & twitter. This morning I got message saying I used 90% of my data! I deleted twitter this morning as well. My friend told me that on at&t you have to use wifi for apps like twitter & instagram. What is going on! & my other question… I'm not suppose to have a twitter. So if I go up to the at&t store, will they be able to see the apps that I've downloaded?

The reason why it got so high, so fast, is because both of you are using it, and when you're loading a website with a lot of pictures, it blows your data through the roof.
Pandora, or anything large, will do that.
I personally hate data limits.

I think pandora uses a LOT of data, so that might be it.

You have twitter so it is probably that readon