Sudden sidebar problem -- goes full width of page

Hi -- I refreshed my page cache today and suddenly noticed that some CSS borders I had (to underline the titles of the widgets in my sidebar) were going across the entire page. I turned off the border (it was attached to my CSS style for .h4) for the moment while I fixed problems. But now I'm noticing that a) when I choose the sidebar title in Firebug, the selection box is the full width of the screen, and b) when there aren't comments filling the content container, the sidebars are wrapping all the way to the left side of the page. (You can compare the sidebar ads in my homepage, http://corporette.com, with the wrapped sidebar ads in this post with no comments: http://corporette.com/2012/11/16/weekly-news-update-94/)

Help! Turned off all plugins and can't figure out what's wrong. I have made small cosmetic changes to my CSS but to the .h4, .navigation, .categories, and .tag widgets -- mostly just color and margin changes. I did add "display-block" to the .categories style, though.