What happened is common. When you use CSS to build a template, the wheels fall off if you make the tiniest changes.
This is why the arrogance of CSS annoys me.
What is happening is that the browser is wrapping based upon the fact that the column wont fit the width defined by your CSS file for the sidebar area. So the whole column gets wrapped to the next place it can be deposited, the bottom of your content area.
Narrow the width of the content area, and the sidebar areas of the CSS file. If the sidebar is too wide, it pops out of the area defined by CSS - so CSS is weak in this regard. May need to clean up the shitty WordPress theme and fix it with position absolute statements in the CSS file.
Also, if WordPress stuffs a shitload of div center commands at the bottom of a post, you'll get this problem also. Go into the posts and clean out the excess div center commands that WordPress uses for carriage returns. Or too many p commands also. The wordpress programmers can't even think of using something as simple as a BR.
The highly inefficient use of a "div center" command is proven when you can just use the old CENTER or LEFT, RIGHT HTML commands - less page code, less crap to reinterpret in the browser.
I cannot believe how sloppy the code is in WordPress.