I'm certainly no expert with regard to this sort of thing. But here's my thinking with regard to what may be going on here based on the work I used to do with injured workers. (I spent a considerable amount of time sitting with clients in neurologists' & orthopods' offices.)

Back pain in general is frequently related in some way or other to the muscles that support the spinal column. To the degree those muscles are not strong, it leaves the door open to muscle strain & injuries to the spinal column itself such as bulging or even ruptured discs. (I actually have one myself.)

If the muscles in your back are not strong to begin with, & then you're fasting (which weakens the whole body at least temporarily since it's not getting any new fuel to operate on) perhaps the combination of not having well developed back muscles to begin with, & then stressing them further via fasting, is leading to the back pain you're experiencing? And then, of course, if you're spending large amounts of time at work (& possibly at home as well sitting... perhaps in less than ergonomically correct positions) that's only going to add additional fuel to the fire, so to speak.

Here again, I'm no expert with regard to any of this. So it's all just my personal opinion. But I do think it would be wise to take the pain you're experiencing seriously. There may not be anything a physician could see on an MRI or a CT scan at this point. But the pain is telling you something's amiss. And the fear I would have would be that, sooner or later, what is now simply causing pain your doctor can't find a reason for could end up resulting in one or more bulged or even, heaven forfend, ruptured discs. At least these are my non-professional thoughts with regard to your post. Perhaps what may be called for here is some physical therapy services to strengthen your back muscles, or even a pain management program, along with some professional dietary services?