I suspect you've accidentally set your input method settings to something you're not used to. (Did you press command-space or option-command-space recently? That cycles through the list of ones you have enabled.)
Do you have a little flag icon in the top menu bar, near the right?
If not, you can turn it on by going to System Preferences > International, click on the 'Input Menu' tab, then make sure the 'Show input menu in menu bar' is checked.
If you click the flag icon, you should get a menu with a list of the input methods you've enabled, one of which will be checked. Try selecting some of the other ones to see if you still have the same quote behavior.
For example, Apple's 'U.S.' and 'U.S. Extended' don't show that behavior, but the 'U.S. International' input method I installed myself does.