Most likely it is your yarn. Are you using a singles (a single ply yarn)? Whether it's a singles or not, it sounds like the yarn has a bias.
http://techknitting.blogspot.com/2011/01/avoiding-yarn-twist-why-does-it-matter.html
But this is actually the article I was thinking of. The photo illustrations are quite good and the article is informative:
http://knitty.com/ISSUEfall05/FEATwhyply.html
--- edited to add ---
Center pull balls can add a bias to yarns too. Here's an article on that:
http://stashkitty.com/yarn-twist
If you do use a center pull ball, or yarn "cake," I recommend pulling from the outside instead. Then, if you start seeing some twist creeping into the yarn, all you have to do is turn the cake over to put a stop to it.
--- edited to add ---
Knitting through the back loop, for a western knitter, will create a twisted stitch, not a stitch that leans with one side wider than the other.
If you guys wanted to see what this looks like, you only had to look at the article that she said looked exactly like what was happening. ie http://knitty.com/ISSUEfall05/images/PLYBRLambKnit.jpg
And hairbender, I guess this proves that not everything you need to know about knitting was covered in your trusty knitting class. The photo is proof that some stitches do indeed turn out wider on one side than the other. No, washing does not remove fabric bias caused by an unbalanced yarn. Steam blocking may help, but odds are the bias will always show to some degree.
There is always something new around the bend to learn about knitting if you are willing to keep an open mind instead of fixating on what you learned in one class. I learned some new things about color work in Portuguese style knitting that were very interesting just yesterday and plan to give them a try in my next pair of mittens.