This would have to do either with a conflicting width parameter or a padding/ margin configuration.
Can we see a link to your webpage or at least the full HTML + CSS code?
As a result of this, any answers to questions you might pose about layout would always need to begin with an examination of the VALID code. That means you can shortcut what often becomes many tedious steps (as seen in this forum over and over again) by first validating your code with the W3C's validator (http://validator.w3.org) and fixing those errors highlighted that you are able to fix. It is perfectly reasonable to ask here about errors that you don't understand or don't know how to fix.
Once you have your page built with valid code, then we can begin the task of troubleshooting your layout. And again, the best way to do this (and to enhance the likelihood of getting a useful answer) would be to upload your page and its dependent files to some webspace that you control, and include a link to that uploaded page in your post.
Make sense? You will really help us help you by doing this.