There seems to be a bug in the last camera raw update that's causing this on some systems
Let Apple know
iPhoto Menu -> Provide iPhoto Feedback
In the meantime revert to the previous one. See here for more:
Thank you so much Terence. Very easy fix now that I know about it. Had made the software update a long time ago (but hadn't looked at photos since), so didn't realize the two were directly related. Was freaking out that my whole photo library was somehow compromised. Very easy fix, and very much appreciated. Just had to revert to the previous RAW compatibility patch.