Without seeing a working example of the page and being to follow through to the stylesheet, it will be difficult to pin-point the exact css problem. (Hopefully someone will disagree, and provide a solution).
What I would suggest is saving your page to yourpage_bak.php, then cutting (just) the formbuilder code from yourpage.php.
Then check that the pre-existing divs are properly styled to accept full-width content without displacing itself. It may be that you need to adjust the div styles' float and clear, or it may be that you need to reduce the width of your form. Or it may be that the exact placement of the form is the problem.
Perhaps you could zip your pages and stylesheet and upload it here so that someone can have a look?