This sounds likethe result of copying and pasting and suggests there are open HTML tag in one of your posts. For copying and pasting text from elsewhere without using either icon 5 or icon 6 in Row 2 of the visual editor, please see here http://en.support.wordpress.com/visual-editor/#pasting-text and here http://en.support.wordpress.com/microsoft-word/#using-word-cleanly

Typically this is caused by an open HTML tag in one of your posts.

Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”

Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)

If that doesn't work see: http://en.support.wordpress.com/themes/layout-issues/#sidebar and http://wpbtips.wordpress.com/2012/05/19/sidebar-at-the-bottom-and-related-issues/