I have been spending loads of time editing. After exporting, my images are super grainy, as if I'd been editing a destructible file. (I am uploading these images to Instagram, as well as my personal site) When I upload the same files, unedited, from a separate computer upon which Lightroom is not downloaded, the image quality is beautiful.
Please help me figure out what is going on.
You offered not a single clue as to what your experience level is, what you've done to learn Lightroom, what version you are using, what computer you are using, what your export procedures are, what your file types are, what your website requirements are, etc. In other words, you're not giving anyone the opportunity to do more than blindly guess.
Please write in detail what you are doing, how you are doing it and what your expectations are.