You might want to move this question to the Administration forum, as you are more likely to get an answer there. I would check to see if you are running the approved driver for your graphics card. Perhaps a Windows update messed something up?
shut down solidworks
start it up
no files open
tools > options > systems options > performance > software opengl
see if the problem goes away
you're using an unsupported graphics card on an unsupported operating system for that version on an unsupported platform, you'll probably have some bumps. and more as 8.1 and drivers get updated for 8.1. like jerry said, most likely a windows update or similar got installed and caused your problem. updating everything may fix it. but the test above will at least confirm if it is graphics related or not. if it does fix it, update the graphics card driver and hope for the best. if it doesn't, i'd try restting your solidworks settings. if that doesn't fix it, i'd go for a repair and then finally a hailmary uninstall/reinstall.
if you're going to stay on this version for awhile, i'd go back to win 7 to get yourself at least on a supported OS for the version that you're running. but the rest will still cause you problems most likely.
This is, more than likely, a graphics issue. The biggest problem with using unapproved graphics (the GeForce that came with your Macbook) with SolidWorks is how fickle it can be, as you're seeing now. You might try different drivers, but no guarantees.