Create a sticky navbar

This is the site I am referring to: http://www.shahriarnk.com/

I have integrated the JS provided by http://stickyjs.com/ to create sticky menu/navbar in the reference site provided above. By sticky navbar I mean that when user scrolls the page, the navbar will stick to the top of the browser.

Right now, the navbar does stick to the top of the browser but the width becomes almost half of what it originally is, ruining the UI.

So, my questions are:
1. Why does the nav bar width become smaller when I scroll down?
2. What can I do to make the width remain unchanged and equal to the width of the content area.

The site is responsive by the way.