Can't put music on my ipod anymore?

I have an iPod nano, 7th gen, and am running Windows 10. My iTunes version is 12.3.3.17.

Yesterday, I plugged in a friend's iPhone - we're trying to see if I can switch her old phone to my carrier, and I needed to charge it before I could do anything with it, so I attached it to my iPod's charge/sync cable. iTunes asked me several questions about syncing her phone upon plugging it in, all of which I declined, I believe.

Today, when I hooked up my iPod to the computer and tried to put some new music on it, I got this error message:

"[song name]" was not copied to the iPod "[iPod name]" because it cannot be played on this iPod.

I can no longer transfer any music to my iPod, despite having put new songs on there just a week ago with zero problem.

Does this have something to do with recharging my friend's phone? Have I screwed up my iTunes forever?