Constant lower left back pain to where i can't bend down, get up from laying down, or sit comfortably. what is it?

Most commonly: What you are describing is a lumbar paraspinal or iliopsoas strain or spasm at your age. However, there are many other things that can cause this that should be checked by a physician, including but not limited to si joint pain, facet pain, and even discogenic pain that has not infringed on a nerve root yet. If it is a paraspinal spasm, then it should continuously get better each day, keep walking.