In iTunes:
Go to iTunes Store
At Right hand corner, click on your email address
A box will pop up asking for your password
Type it in and press View Account
You will go to Account Information
Click on Purchase History
That's where you can see what kind of stuff you have made
purchases on.
If there are any errors, you can click Report a Problem.
2 likely scenarios though.
1. You bought something without realizing it.
2. You made an in-game purchase (like buying the big eagle in
Angry Birds). The app might be free, but the item will still be
charged.