External monitor view looks stretched

Over the weekend, I ventured into the world of Mac ownership with a new macbook pro. At my office, I've connected it to a 23" external monitor just to make it easier on my eyes. When I first set it up last night, the view on the external monitor looked stretched or a like blown up version of what's on my computer screen. (FTR, I have mirror display turned on because I'm keeping the macbook on a shelf under my desk and just want to work from the one screen.) I messed with the resolution size of the external monitor but that didn't fix the problem.

Today, when I came in and hooked everything up, the screen looked like how I originally expected it to look - not stretched or any of that junk. Just very crisp and clean - exactly the way I wanted it. Then a bit ago, I tried to mess with clamshell mode and for some reason, that made the external monitor go back to being stretched. I've tried restarting several times and unplugging the DVI->mini display port converter but it's not fixing it.

I have no idea what I did right this morning, but can anyone help me? It's a stupid problem, for sure but it's annoying staring at this stretched screen.