Quick question about name change..

Ok so you all know i recently got married in October.

Well I was just wondering.. Am i supposed to drop my last name and keep my middle name?

for example..

name before marriage = Khristine "Middle Name" "Last Name"...

name after marriage = Khristine "Last Name" "New Last Name"?

because for some reason.. when i call my credit card companies, they change my name, but leave my middle name.. so i'm so confused!! someone help me! LOL