But, on to our problem! Sometimes it's simple, sometimes not.
Play Sherlock Holmes and test some changes without saving them until you use Preview and see that the change works and does no harm. It's hard, but I don't know any easy way. Think about the last 2 or 3 things you did. One oddity of the "low sidebar" is that there is sometimes a "delayed reaction" before the visible bad effect occurs, so that the post, image, or link that's the culprit may SEEM to have been innocent (caused no immediate problem) at first.
To test posts one at a time, use the Permalink feature; if you click on the Post Title, your site will show only that one post. When you keep doing the blog like that, you MAY find a single post that pushes the sidebar down and that post will be the cause of your trouble. In other cases, of course, the trouble is coming from the sidebar and you just have to figure it out.
To hear all this put another way, go to Peter's post about it or Chuck's post.