Is it obligatory to repeat the Fajr prayer if one doubts the presence of ritual impurity (najāsa) on one's garment after completing the prayer?

It is not clear whether the questioner was certain of the presence of impurity during prayer and then doubted it, or if she initially doubted it during prayer. If she was certain of the impurity's presence before finishing the prayer, she must remove it if possible. Otherwise, she should cut off the prayer and repeat it after removal. If she did not do so, she must repeat that prayer, unless the impurity is excusable. However, if she doubted the presence of impurity during prayer, there is nothing upon her, because the default state is purity, and it is not nullified by doubt.