Left shoulder pain when reach behind/across body,lay on left side/on back at night what could this be?it's been happening about a month after accident

Do Some Research: This sounds like a "classic" case of a rotator cuff problem. That is, the tendons that move and/or stabilize the shoulder. If you're careful, it can heal on its own, but if there's been no progress after 4-6 months, go see an orthopedic doctor. Before that, avoid straining above shoulder height. Look on the internet for stretching and mobilizing exercises, use Motrin and apply ice after activity.