iPod touch storage almost full, but deleting mP3s won't free up any storage

I'm not very tech-savvy so excuse me if this explanation isn't worded clearly. I used another program (not iTunes) to put mP3 files on my iPod touch. They still show up on the music app.

Anyway, I recently got Spotify because I felt bad about ripping so many mp3s off youtube, so I deleted most of my hard-worked-on music library and switched over to spotify. I deleted the mp3s from the music app and now the mp3s don't show up in my music library. Despite this, no storage space has been cleared, and I don't understand why. How do I fix this?