Back pain near left shoulder blade? Hurt or bad posture?

[Ashley]

I just got my first steady job at a walmart bakery. I'm in my twenties, skinny and my muscles are weak from not being active and from suffering from anorexia when I was a preteen. I can lift 29 pounds boxes, but they're really heavy to me. At my job I sometimes have to lift lots of these heavy boxes, plus the bending and being on my feet all day. I went to culinary school and would have lab classes and even a 7 hour class where I worked in the restaurant, so I'm used to being on my feet a little. But about two weeks ago I started having back pain. It's only near my left shoulder blade. It started when I sit down at my computer one night after work and was sitting uncomfortably because I was in a hurry to look something up real quick before I went and did something. My back started hurting and I immediately adjusted myself to a more comfortable position. But it's hurt ever since then. It's gotten a little better but it's still there. I don't know if I put too much stress on it at work or if It's from bad posture or both. I often find myself sitting with my left shoulder slightly away from the chair while my right side is rested comfortably against it. Since I noticed this I try to remember to adjust myself.