I didn't see you mention where the pain might be, but if you had an MRI and an X-ray and they came back relatively negative, I'm going to take a shot in the dark and guess it's a connective tissue issue.....probably ligament strain.
This shit might not heal on it's own. I have been dealing with a similar issue for almost 9 years now with no progress. If my assumption is right look into prolotherapy. Google it. Look into slipping disc syndrome too, to get a better understanding of ligament issues and how they can cause chronic pain. Muscles heal quick and disc herniations usually have quite a few neurological symptoms to go along with the chronic pain, thus my guess is a ligament issue.
Inversion table or boots have helped
Rumble roller has helped
Heat has helped(wet heat is best i.e hot tubs etc)
But these are just relief of symptoms, not a cure for the main issue
That's where prolotherapy comes into play, also know as regenerative injection therapy. This might be your saving grace! Hopefully this helps!