I have had the same issue for years now. It started before I began riding, and has gotten worse.
I have had multiple tests done (bloods for things like rheumatoid arthritis, inflammation markers, etc), xrays, etc by Neurologist and Specialist Physician all coming back clear.
I have been to Biokineticist's, Physio's, Chiro's, etc and only found temporary relief.
I was eventually told by the Specialist Physician (and a Bio prior to that) that the pain is probably caused by my posture and lifestyle (I sit many hours for work, driving and desk), and that although the mountain biking probably hasn't cause the issue, it's not helping.
I was told to strengthen my core significantly, and stretch frequently, preferably by yoga or pilates.
I have recently started the process of trying to strengthen the core, and do some yoga stretches after every ride. No change yet but its early days.
Hope this helps
