No, I can't guess what it might be nor would any hunch be more than conjecture.
As bad at looking at the whole person and all their circumstances as is modern medicine, rather than, like a machine, as the sum of certain inputs and outputs, what can be said positively, within their ovuer, is that medical diagnostics are pretty good.
I'd encourage availing yourself of some medical-model (vs. whole-body or holistic) feedback as an aid (Not necessarily THE only answer) to diagnosing this; don't mess around...you can always fast, later.