help... my animation world is getting turned upside down!

I doing an animation and the object is a plane, its was built to scale. I’m now starting to let take flight in animations so the distance it is flying is also to scale (if that makes sense) basically I am working in a huge Blender world.

But I’m having a really anoying problem occuring occassional (using 2.65a)… every object in my animation will all of a sudden turn completely upside down (z axis pointing down).

This does NOT occur in camera mode… only when using the front view sideview top view. I have tried to flip back and forth from perspective mode and back to orhograhic. One time I was able to select an object and go into edit mode, than back to object mode and it fixed itself.

Even turning blender off and restarting doesn’t correct the problem…

any advice would be greatly appreciated because this is driving me crazy… Thanks