iTunes Artwork Suddenly All Mixed Up!

I’ve spent countless hours over the past few years manually getting artwork for the many songs I have that arent’ in iTunes’ database. Now I’ve noticed, though, that the artwork is attached to the wrong song. At first I thought it was just a few songs, but I noticed today that more of them are wrong than right. Something has actually changed what is embedded to the songs, though. If I view the properties of the song outside of iTunes, the artwork shows up wrong there, too. Any idea what could be going on and if there’s a way to fix it?

Thank you!

Also, I’m not sure how this would affect it, but the only change I’ve made recently is that I installed the FoxyTunes extension for Firefox the other day.