It's an interesting piece to translate because in many of the existing versions the menstruation aspect is ignored. Instead the texts tend to imply she needed to urinate which, in my opinion, changes the tone of the passage considerably from that of a woman with an uncontrollable physical issue that she must deal with to one who simply can't hold her bladder and puts her whole army at risk. However the words 'fúal fola' together seem to be an idiom for menstruation*, 'blood urine' or 'blood water' [fúal = urine or water, fola = blood] There may also be some cosmogenical significance to Medb's creation of the 'three trenches' with her menstrual flow.
The original Old Irish is as follows:
Is and drecgais a fúal fola for Meidb, .i. sciath díten dar éís fer n
There it was advanced on Medb her bloody fluid, that is [then she said to Fergus] "bring about a safeguard covering the troop of the men of Ireland