Aunt Flo (AF) is another way to say menstrual period. The term was coined in reference to unwelcome family members who show up unexpectedly for a visit.
In the fertility community, sometimes AF is positive as it can be required to initiate infertility treatment and other times AF starting is devastating as it means treatment has not been successful.
FertilitySmarts explains Aunt Flo (AF)
The term Aunt Flo is used to refer to a menstrual period when someone does not want to overtly state that they have their period. This may be used in public situations when someone is feeling embarrassed about saying the word "period," or to appear casual.
An example of a sentence using this term is: "I don't want to go swimming today because Aunt Flo is in town."