A Bum-Stress (n.) is a synonym of a "sulk", basically meaning 'getting annoyed for a really stupid reason.
Related:
bum stress Pissed Off
Girl: STOP IGNORING MEEEEEE!
Boy: What are you talking about?
Girl: You aren't replying to my texts! DO YOU HATE ME OR SOMETHING?!
Boy: I forgot to reply to ONE text! Stop getting in a bum-stress about it...