I have occipital neuralgia, sciatica in both legs and have 40% rating for my back. So I am in a pretty good amount of pain throughout the day. It gets worse throughout the day. Chronic pain is typically tied to an injury. That's why I was asking what yours was. You need a diagnosis, nexus and service connection for direct service connection. For Secondary's you need a diagnosis and a link leading one injury to the new injury. Example being: Your back causes you to walk differently and because of this you now have hip issues. etc.
In your case you could request for an increase in your rating, or turn in a supplemental claim to try to raise your PTSD because it is exacerbated by your back pain. Which makes sense for most of us. Just keep in mind they can lower your rating as well when you do that. That choice is yours alone to make. But as far as chronic pain being its own rating.....I don't think that's the case. There are things I know that are claims for instance fibromyalgia but I don't know anything about how that is because I don't have it. There is also a number of radiculopathy claims if you have those issues, but again need a diagnosis and link with current direct service connections for that.