Abu Sa'id Al-Khudri narrated that the Allaah's Messenger (PBUH) said:
"When anyone of you is in doubt about his Salaat (prayer) and does not know how many he has prayed, three or four (Rak'at) he should cast aside his doubt and base his prayer on what he is sure of. Then, he should perform two prostrations before Taslim (salutation). If he has prayed five Rak'at, they will make his Salaat (prayer) an even number for him and if he has prayed exactly four, they (i.e. two prostrations) will be humiliation for the devil."
[Reported by Muslim, Bulugh Al-Maram, Ch.8, Hadeeth No.265].
It is normal that sometimes we lose count of the number of units during our prayer. This may be the result of not paying attention and thinking out of prayer or the devil's interference. According to Sunnah, whenever one is in doubt as to whether they have prayed more or less units of prayer, it is always proper and better an option to choose what is less. And then pray what remains.
After completion and before saying the Salaam, one should perform two (sujud) prostrations. This is to fix the anomaly of doubt that arose in their prayer. Learn one new thing about your Deen every day. And Allah knows best.
If during a fast a woman comes on her period, she may eat thereafter. She will have to do Qadha of this fast.