I think you should monitor the tendency a little more. Now I'm blessed with very little hassle from my periods myself, and my bad days are generally more linked to nutrition and sleep, so I think I would take these factors into consideration too before blaming it all on the period.

You didn't mention any other potentially influential factors, but could it have something to do with stress (crazy drivers while commuting, perhaps? ), several days of too little sleep or perhaps less than optimal nutrition either race day or the days before? I'm not saying that is has nothing to do with your period, and, having few period-related problems myself, I may have little understanding for how the menstrual cycle may influence athletic performance! However, my boyfriend's a long-distance triathlete, and he may be completely taken out by something like a bad night's sleep or too little food. If this race was on the border of what you're accustomed to I think there might be something like that is more influential. Monitoring things like this is really why one should have a training log, but personally I never manage to keep it up...