someone stole my cc info but how?

[lemontree]

a few days ago i discovered that multiple fraudulent purchases were made on my credit card. this is what i found out:

the fraudsters have my name, address, credit card number, my credit card's security number, and possible my cellphone number (because the number they used had the same area code as well as the same first 4 digits. in other words 7 of the 10 digits are the same).

i am racking my brain in trying to figure out how they managed to steal my information. i'm wondering if you guys, especially the computer experts, can give me some suggestions.

now, i never throw away my credit card bills. when i throw away mail i usually tear up my name and address pretty good. i do make online purchases but i mainly use paypal. i use my credit card at stores as well. I made more purchases than usual at stores last month because of Christmas shopping. that said, I think it was still under 10 transactions? also, i did have to enter my credit card number twice. but the second time, as i would find out later, was after the first fraudulent purchase took place. i did have to enter the security number of my credit card for both orders.

whenever i have to enter my credit card info which is really rarely, i always run every scan i have before i do so. i know that probably isn't 100% effective but still.

i'm just wondering, do you guys think the chances that my info was stolen from my computer/the internet, or at the shops i went to? and all the stores i purchased from are big chain stores. what do you think?

and if the info was stolen when i was making purchases online or from my computer, what should i do?