Must prayers be repeated after discovering a drop of urine had descended during menstruation and before the prayer, given the prior belief that it was a delusion?

Your previous prayers are valid, and you are not required to repeat them, as doubt regarding an act of worship after its completion does not affect its validity. As for the drop of urine, if it is merely a doubt, do not pay attention to it. If you are certain of it, do not perform ablution until all the urine has ceased. Then, cleanse yourself (istinja (definition)') and perform ablution for prayer. You must disregard obsessive thoughts (waswas) and not pay attention to them, because succumbing to them leads to great evil.