Volume going up and down through songs

[confederate_gen]

I have a S6 Active and within the last two weeks i have noticed that the volume will constantly go up and down through the whole song. This only happens  when I use Spotify and not when I use any of my other music apps.  It also happens whether or not I am listening to it with earbuds or have it plugged into my car's AUX port. I thought it my be because i'm a basic user and switched to premium and it's still happening..