Why does music keep going back on my iPhone?

I have always kept my iTunes set so that it doesn't automatically sync my entire music library to my phone, and I have never had any problems with it until now. Now when I try to just add one song to my phone, it syncs my entire library to my phone, even though I don't have it set to. I can't get it to stop, and every time I want to just add one song, I have to go through and delete hundreds of unwanted songs from my phone after. How do I fix this?

This is because your phone number is stored on your iPod and your iPod settings allow it to sync. Change the settings on your iPod and delete your number from your iPod and it shou
D be just fine.

What the heck is previous answer? Ignore him.

Updates to the new iOS have really annoyed a lot of people. If you look at the song list on your phone, do you see a cloud icon next to all the songs?
The cloud icon means the song is in your library, but NOT on the phone.
How to fix?
1. On your phone, go to settings - Music - and turn off the switch that says, "show all music". This prevents music stored in the cloud from showing on your device.
2. Now go to settings - general - storage - music, tap on music to select. Now touch and slide from right to left, tap red delete box.
This removes all music from the phone.
3. Now you can sync only the music you want from your iTunes library without cloud storage showing on your iPhone.

If this is not the case. Check your iTunes preferences to see is auto sync was turned back on, or if sync entire library, or fill empty space with music has been checked in summary page.