You don't have belly pain, right?
re constipation:
Try laxatives that can be used long term and do no cramping, just stool softening (like Movicol, not senna please)
Try probiotics - and I am sure you have.
Stay well hydrated (pain in the patella if one is not thirsty, but that's the point)
If you think the belly buldge comes more from bloating (winds),
you can try malabsorbtion breath tests/celiac blood or different elemination diets.
(Can't see if you feel bloated or as to why the belly)
There is one thing that my skinny (but little belly having) daughter has,
that I note here for you therefore (and might be completely irrelevant):
She has a prolapsed transverse colon,
it hangs down way below her belly button. As if it was far to long.
So no matter how skinny she is, her belly will have a little bulge below the belly button since almost all her small and large intestine sits there.
Just by pure arrangement of guts there is a belly.
She is too sick yet to try any 'training' for belly muscles and yoga for organ lifting, that are meant to lift it a bit.
From colon wash I stay away big times,
but constipation is deffinitely to be avoided anyway and especially if colon was prolapsed which is a catch22. A prolapsed colon tends to more contipation. Duh.
Have you had a look on your x-ray or MRI/CT where your transverse colon sits?
It is always said to cause no problem re pain
(I am not so sure since my child is in huge pain down there),
but it is agreed to that simply the 'mass' of guts in one spot plus colon prolapse causes some visible belly even without bloating.
Just a little tale from our experience just regarding skinny, but not around gut area. (even you didn't mention, if all of your gut area is 'bigger' like 'beer belly' or only below belly button like a buldge)
Autoimmuneantibody screen (ANA) and thyroid are sure tested, right?
Hm, difficult one.
Let's see, if someone has very similar symptoms.