Bad Posture - Neck and Chest Pain

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I have very bad posture when sitting. I am often hunched over without really thinking about it. I also tend to lay on my left side with my left arm propping me up and my head right against my shoulder (sort of hard to explain), though not as often as before. Sleeping on my left side is the most comfortable usually, with my left arm up under my pillow. While drawing or concentrating, I noticed my neck becomes very tense.

Here's what happened recently. I started having pain on the left side and back of my neck, and then headaches. I assumed it was due to tension, so I tried relaxing more throughout the day, and changed my pillow. My friend's mom also gave me some smelly stuff to put on it, which helped as well. The neck pain went away, but now I have this odd chest pain. It's like right above my left breast, almost going into the armpit. It happens only while sitting or lying on my left side. The pains disappear when I consciously sit up straight. There's a muscle right there, so I wonder if it's tensed up and hurting because of my slouching.

Anyone got any tips for helping that muscle, and to improve my posture?