Check all the parameters in your Character panel. Photoshop remembers the last settings you used.
Well... that was easy... I don't really play around with the panels very much. Maybe I should?
This gets me, from time to time. I work on a lot of DVD Menus (specialized PSD's), and quite a few have rather odd Character parameters, like All Caps, and some different Kerning. Then, I go to a regular Image, and have to do a bunch of resetting, as the parameters from those Menus "stick." I quickly learned Reset Tool.
Good luck, and it IS easy, when one knows where to look, and what to look for.