Slight echo sound on music track

Hello!

Earlier today i recorded bass playing over a song. It sounded good, just like any other time (over the many years). But i decided to record that same song again and something weird happened.

The music track on the background sound slighty echo or like two song top of each other and not synced. When i mute my bass track, music track sounds normal. Soon as i unmute my bass track, it sounds echo. I dont know what i did wrong.

The i tried convert music to “mono” and that seems to help. But in earlier in my recording sessions it didnt required to convert mono…

Any ideas what happened?