Strange volume behavior

[anon(9405500)]

Hi, I was watching youtube videos with the Volume almost full, still it wasn't too loud, thought the videos are quiet. After watching some with the same audio level, while in the middle of a video, I received a Whatsapp message, which obviously interrupted the sound for a moment, and after, the volume was way louder on the video. Although the volume setting was the same. I was using the YouTube app, not watching from browser. Android Version 5.0, the latest for Galaxy S5. Can this be physical problem (the phone vibrated when receiveing the message), or maybe Android didn't realize that I was setting the volume up and got stuck somewhere? . Thanks for help!