So it is not during/right after your period that you're bonking, but beforehand?

For me the most bonk-prone time to ride is right afterwards. My body apparently really misses the oxygen-carrying capacity of those red blood cells that I lose. For a while I was using BCPs to help keep periods light, but the hormones gave me migraines so I had to stop.

When is your century, are you pretty sure it's going to fall into your bonk-prone timeframe?