I have my srenght raised with draw upon holy might with two points but the strange thing is that my thac0 goes up!
it goes from 0/-1 to 5/4

what is the cause for this?

I'm not 100% sure but is it possible that draw upon holy might raises your dexterity too ? I know it raises con aswell as strenght but i'm not to sure about dex.

It raises Contitution, Strength, and Dexterity by 1 point per 3 levels of the caster, up to level 20 (well, 18 since it's by threes here). So you can raise it by 6 points each at max, which is quite a lot. My ToB Sorceror gets 25 strength and 24 dexterity from casting it...not like it's much use to him anyways ;)

That's.....bizarre. Give us details. Your stats, weapons, if you're using any STR boosting items, any other spells in effect, etc.

i have a belt of hill gigant strenght, but the stats show the modified version from DOHM

Remove all equipment and spells, check your THAC0, cast DuHM, and check your THAC0 again. Then put your gear back on, one piece at a time, and find out what precisely causes the THAC0 loss. A STR girdle and DuHM should be cumulative, no matter in which order they were applied.