Wa alaykum salam wa rahmatuLlahi wa barakatuHu,

It is permissible for her to fast sunnah fasts before completing the fasts missed due to menstruation or any valid excuse; yet if the excuse is not valid, then the missed obligatory fasts must be performed immediately. She must not delay in making up missed fasts until the next Ramadan. (I’anat al-Talibin v. 2, p. 395) Yet, it is disliked to perform supererogatory fasts before one has made up the missed obligatory fasts. (Nihayat al-Muhtaj v. 3, p. 208)

And Allah knows best.