did this happen after doing a keystroke shortcut (like ALT+SHIFT+DRAG)? or did it just crop up?

try tossing your PREF file and see if it clears up...

 

to delete and re-create the preferences file, start Photoshop and, as it starts, hold down Control+Alt+Shift (Windows) or Command+Option+Shift (Mac OS), and then click Yes to the prompt "Delete the Adobe Photoshop Settings file?" 

let me know
:)
retrocity