[Sneaky]

Hey RVT,
you havent had her too long have you?
Cant remember exactly how long ago you got her,
but its gotta only be 6-8 months ago right?

I would suggest testing the water for ammonia,
nitrItes, and nitrAtes.
Do a good water change to make sure its nothing in the water.

To me, it sounds like Old Age creeping up on your little friend.
Bettas can live from 1 year to 2.5 years which is pretty much max,
and most bettas dont live past 1 year of age.
With no noticeable marks, spots, or wounds, I can only suspect one
of three possible things:
A) Water quality issue, test that water and see whats going on.
B) Old Age
C) Internal Bacterial Infection of some kind.

After you test your water (or have it tested somewhere), post back the results here.
If the water is good, (ammonia 0, nitrItes 0, nitrAtes below 40),
then its likely one of the other causes.

If you wanted to try treating her with something, if you suspect it may be a bacterial illness, Id suggest the medicine Maracyn 2. It cures most gram negative bacterial infections. You can pair it with Maracyn (1), which cures gram positive infections, if you want to cover the board.

I cant fully remember, what other fish are in the tank,
but I seem to remember it is 10g right?

One other thought, have you added any new fish lately?
Any small fish like neons or whatever in the tank that may have died
and could be causing an ammonia spike? Check for bodies in plants and stuff.

Also, have you noticed her to be pooping at all? normal poop? white,stringy poop? Any bloating at all? Constipation can commonly affect these fish, so try taking a canned pea, rinse it, take the outer hull off, and try feeding the soft inner pea to her. Do this for a day or two with no other foods.
If its constipation this should help. If shes still eating, its best to try now before she starts refusing food. Most bettas love peas.

Anyway, keep us posted and good luck! Hopefully one of my ideas may
spur something on your end.

Get well little Bertha!