Wondering what could be causing the discomfort I'm having on my left side of my chest, Hurts when I slouch, and bend my neck down really far..

Sounds muscular. I'm sorry you are having pain. Given that you experience the pain when you bend forward or slouch, that suggests the pain has something to do with your muscles and bones, rather than your internal organs. Did you engage in any unusual physical activity recently? For a proper assessment you will need to consult a clinician for a full evaluation. Best wishes.