Exporting Photos to my Desktop from Lightroom

Help!  I can no longer export any of my edited photos from Lightroom to my desktop.  I'm not sure if something has changed in an upgrade...???  I'm a relatively new user, and don't really understand a lot of the technical aspects to it, but every time I try to export, the photo looks great, and as soon as I export it, it becomes completely pixelated throughout.  I've tried to export in both small and large jpeg format.  The same thing happens.  I even exported a jpeg that I had not edited at all (other than importing it in from RAW format).  And it was full of pixelated lines (photo attached).  So I'm not over-editing.  What is happening?  This is essentially making Lightroom almost useless to me!