lowering sugar ( Tea without Sugar ) made me dehydrate

Believe that you are using the forum correctly. 

There is no way that tea without sugar will dehydrate any more then with suger. Tea alone will dehydrate you because "Tea" is a "Diuretic", as is Alcohol, Coffee, and Pop.

Hm, interesting observation.

Which sugar (normal household table sugar = sucrose, or sorbitol)

and

which tea?

Sorbitol is known to have a laxative effect,

with household sugar (sucrose)....people swear by its laxative effect for babies since they are not fully equipped with digestive enzymes and the sugars seem to pull fluid into guts (like sugars lactose, fructose do for adults often),

but for adults sucrose usually has the oposite effect of rather causing constipation than relieving it. Especially fructose malabsorbers (who get diarrhea), take houshold sugar, since the glucose in it balances the fructose out to avoid diarrhea.

 But there is another factor: caffein is known to contribute to constipation.

So if you drink non-herbal, but 'caffein' containing black tea (that is mildly diuretic),

you might have had some impact.

It is known, when suffering constipation, that one should rather drink water, decaf coffee or herbal teas, but not black (green teas have less), rather even sugary sodas than anything with coffein.

But there is so much we eat additionally, live circumstances, that could lead to constipation or there nerves in guts inself being influenced, that I think it is hard to pin down the culprit that easily. (?)

Honey has more fructose in contrary to table sugar and hence pulls out more liquid into intestine/stool. (honey is a mild laxative; fructose malabsorbers hence avoid it completely)

You could really do some trials, 

different tea, with without household sugar,

different tea, with without honey or sorbitol (sweetener).

Hydration is key, so yes please drink well, have soups, but if constipation hits, it's sometimes hitting also well hydrated people for other reasons.

All the best!!!

Enjoy your tea!