Data usage cap popup

Hello all - I would like to know why I pay for my internet and my data usage went over the cap.  I received an email from COX stating this, which is fine.  However, my mom who is living with me and is not on my account, while she is surfing the internet on her netbook, she received a popup on her computer that was addressed to me and stated the same thing about my data usage cap.  The popup is not one that you can close.  It covers a third of her screen.  Why is she getting this popup on her netbook and it gives my name and my internet data plan account information.  She doesn't sign into COX, she doesn't have my account information, she was on her doctor's website and all of a sudden, this popup came up.  WHY?

David