Songs Disappearing from Playlist

I started adding Christmas music to my device a week or so ago by going to my "Christmas" playlist and checking some of the songs. (I have my iTunes settings so that unchecked songs are not synced to my device.) Last Sunday, when I went to add more, I noticed that all of my songs in that playlist were checked... and that now, in addition to my Christmas playlist, I have playlists titled "Christmas 1" through "Christmas 5"... and that 1-5 seem to be exactly the same, while "Christmas" has one additional song... and that no Christmas playlist has more than 118 songs. (I love Christmas music; 118 is a small fraction of the Christmas music I have downloaded.) So, my guess is that the songs I checked over a week ago are the ones still in the playlist, while the others have disappeared, for some reason. At least some of the unchecked ones are still in my library, so I would guess they all are.

I don't understand why they disappeared. I've suspected it's happened before, but not on this scale. I also don't understand where the extra 5 Christmas playlists came from. I didn't make any changes to playlists from my device in that time, which is what I understand causes that to happen.

I like my music to be organized (especially when it's seasonal), so it's really frustrating when I have to find the time to do it over and over again because it's been screwed up somehow. And with other playlists, I might not notice which songs have disappeared (just that I swear I had more songs on that playlist...), so, again, I feel I would be wasting my time by trying to improve them.

Why are my songs disappearing from my playlists? Can I get them back? (Do I need to restore a backup of my device?) Is there a way I can prevent this from happening again? Is there a better way to organize my music?