describes emotional status of person who is not feeling good and who can't specify what exactly is wrong. It comes from "It smells like pretty big shit" > "pretty big problem".
Sometimes you can use this phrase when you have problem with your defecation, to stop it, and you would like to aware other person from social degradation.
Bob: I have shit feelings today.
Sash: Why? Because of school?
Bob: Maybe... but during all day I did nothing.
Bob: Boss sent for me and I am afraid about my future.
Sash: But you don't know what is going on!
Bob: Shit feelings... maybe you see me last time...
Bob: Oh no! Shit feelings!
Sash: Good Lord! No toilets around. I have to go! Sorry man.