Keep in mind that divs are block level elements and p tags are inline. When adding borders to a div – you have to account for this space (i/e: a 1px border on the left and right side of the div equates to 2px). If this is not accounted for it can change your layout.
It’s much easier to see the issue with your code. If you can supply this it will be easier to get an idea of what is occurring.