My periods stopped about a year or so after I started riding; coinciding with the purchase of my first road bike. I remember feeling just awful in the days before it started and during the first two days. I could barely turn the pedals over and my average speed definitely was lower. I also remember thinking that if I was competing, it would totally suck if I had my period. When I taught fitness classes, i would pray that i didn't have to teach the day before it started or on the first day.
But, I had horrible PMS, my whole life (the physical symptoms), from age 11 on, and I never felt good at the beginning of my period, although I only had bad cramps when I was very young. Menopause was a breeze compared to my period; I was either getting it (10 days), having it (5 days), or trying to avoid thinking about getting during the other 2 weeks of the month.