Odd problem with MP3 playback

Sonar X2 Studio user - Allen and Heath ZedR16 mixer - Sweetwater Creation Station computer (which is only used for my music projects)
 
I've created MP3 files of my music projects before without a problem, tho' it's been awhile and I may have forgotten a simple step, but I'm perplexed: 
I just recorded a simple vocal track (getting into voice-over work and have a need to do demos/auditions from home).
Only effects I used were the Sonitus compressor and EQ.
The track plays back fine on my recording rig, in Sonar.
Exported this track to create an MP3 file - 16 bit; triangle dithering.
Copied the file onto a thumb drive and loaded it onto my main home computer which has an internet connection so that I could e-mail the demo to my producer in another city.
Before e-mailing it, I played the demo using Itunes and here's the problem:
All the low end (sounds like sub-woofer material - I don't have a sub-woofer on this computer or my recording rig) is coming out of the right speaker of this computer and everything above the sub is coming out of the left speaker.
At first, I thought only the left speaker was receiving a signal, but I unplugged the left speaker to isolate the right and that's when I heard nothing but all the low end coming from the right speaker.
Once my demo track had finished, Itunes shuffled to the next track and it played just fine, so there's something I've done wrong (evidently) in exporting the raw track to an MP3 file, but I can't think of what would cause this.
Any ideas? 
Please let it be something simple - I don't mind feeling a bit foolish!

Hmmm...well, there's only the option to Export Audio, as far as I can recall (not sitting in front of DAW at the moment), but thanks, I'll look again.

The only audio I was exporting was a single track of voice-over for a commercial. Not sure what you mean by 'selecting the correct source'.
All of the audio did export, but once I played the MP3 in Itunes, I had the problem described above, where all the low end was coming through the right channel and everything above, like 80 Hz was coming through the left channel.

You do not by any chance happen to be in surround mode?

Thanks for the suggestions, guys - I figured it out!
The mistake was made during export. Under 'Bounce Settings' at the bottom of the Export window, I needed to change the Source Category to 'Tracks' instead of 'Entire Mix'. I then listed the single track where my audio was recorded under 'Source Buses/Tracks'.
Seems life was easier when I was editing tape with a razor blade!  ;)