Which muscle is located to the left of your spine, in the middle of the back? how do you differentiate between a pull or tear? laying down hurts!

Several: There are several muscles in that area that form approximately three layers. The most superficial group include the trapezius and the latissimus; the next level deeper has the muscles that primarily extend the spine, and then the deepest level contains the rotators of the spine among others. In general, a "pulled" muscle will get better more quickly than a tear.