Why Does My Chest Hurt When I Lie On Either

Why does my chest hurt when I lie on either side? The pain is more prevalent on the left side and hurts the most when I exhale completely. It lasts for scary lengths of time sometimes and it's much more common now than before. Sometimes now, I can't even lie down at all. I have to prop myself up in a reclined position. I'm 24 years old now. I'd say I have been feeling this pain since I was at least 19. The older I get, the more painful it gets and the longer it lasts.