When changing album artwork, my song length changes and repeats.

I'm reorganizing my iTunes library and I've noticed that when I try to change the artwork for a song/album, the songs are affected. The song becomes longer, but the extra space is the song repeating. For example, a 3:30 song will become 4:00, and then when the song is over, it'll fill up that extra 30 seconds by going back to 3:00 and replaying that over. It's affected nearly my entire library, rendering everything damaged! Most of this music is from CDs, so I've had to reburn everything. Does anyone know how to prevent this from happening or fix it when it does??