Back is really painful since car accident - over a year ago. Constant ache&in a lot of pain having to use a hot water bottle. What could it be?

You need a diagnosis. If you continue to have a "lot of pain" a year after the accident, you need to see your dr about this and have an MRI of your back to see why it is hurting. From there, you may need to see a physical therapist or a neurosurgeon, depending on what the MRI shows. It is my opinion that you should not see a chiropractor about this, unless you know with certainty what the underlying problem is.
See a chiropractor. I recommend seeing a chiropractor so they can help you.