Must one repeat the Dhuhr prayer if the worshiper doubts that his trousers became stained with blood during the prayer, and he is unsure whether he changed them or prayed with them on?

Not remembering whether you prayed with blood-stained trousers does not affect the validity of the prayer, because the possibility revolves around two matters by which the prayer is not invalidated if the blood is minimal. In addition, doubt after completing an act of worship is not taken into consideration, and therefore does not affect its validity, as mentioned by An-Nawawi.