Is the discharge that comes from a woman when she thinks about her fiancé's words – even if they are ordinary words – madhiy (pre-seminal fluid)? And what should she do regarding her prayers and Qiyam al-Layl (night vigil prayer) if she suspects the continuous discharge of fluids during prayer?

The questioner is congratulated for her acts of obedience and urged to disregard obsessive thoughts. The discharge due to speaking with the fiancé is likely madhiy, which is impure and invalidates ablution. Vaginal moisture (rutubat al-farj) does not invalidate ablution unless it exits to the outer vulva. One is not required to inspect for it in case of doubt, as the default is that it has not exited. Repeating ablution many times for Qiyam al-Layl because of this moisture is not prescribed and is a result of obsessive thoughts. The fiancée is a stranger to her fiancé before the legal marriage contract, so it is not permissible for them to speak to each other except for a religiously valid need.