Check with Apple make sure you have the latest software for your Mac model. It may be a display driver problem...
Update: don't know what happened, but it seems to have solved itself. The two suggestions I tried from adobe's customer care twitter were:
1) resetting my photoshop preferences, which failed to fix the problem
2) Preferences>performance>uncheck "Use graphics Processor", restart Photoshop and then recheck. This also failed
After a few hours, I reopened photoshop and suddenly everything was normal. I didn't do anything to it after the second suggestion, so I still don't know how it was fixed.