The Defense Finance and Accounting Service (DFAS) has posted January 2013 Leave and Earnings Statements (LES). I know that a lot of people have questions about the amount of their mid-month pay, and hopefully the LES will help clear up whatever has happened.
Everyone will have three changes on their LES: an increase in base pay, an increase in Basic Allowance for Subsistence (BAS), and an increase in FICA - Social Security withholding. These change should roughly even each other out. (Very roughly.) Also, if you carry Tricare Dental coverage, the family premium has increased by $1.07.
Some people will have increased Basic Allowance for Housing (BAH) but, due to the rate protection program, almost no one should have a decrease in BAH.
If you've made changes to your withholding or Thrift Savings Plan contributions, obviously that will affect the amount of net pay that ends up in your bank account.
If you've read your LES, and what you've read doesn't make any sense to you, ask questions in the comments. I'd be glad to help!