check with your Doctor. I don't know your medical background and certainly your doc would do some bloodwork to make sure all is well ie. the liver.

Agree, getting the blood spots checked out would be an important step.
There is some research to suggest that Psoriasis is linked to your nervous system. The majority find that their outbreaks are pretty symmetrical in terms of where they are on the body which would support this.
Maybe, given the nerve density in your spine/back you are just feeling it there.
Could be that the skin there is actually a bit dry, so maybe a plant based moisturiser could help.
No kind of expert but have read a lot about P, just suggestions.