Lines adding by themselves

[chavic]

I am working on a document in Word 2007. In formatting headings of sections, with centered headings and left justified paragraphs, when I am indenting the first line of the paragraph, a line all the way across the page will insert itself for no apparent reason. If I delete for formatting, the line will delete itself, but the format is required for the school paper I am doing. How can I delete these lines and make them stay gone?