Overtraining, evening exercise, and the full moon - definately all good possible answers to the "why". Exercise elevates your heart rate, and after a particularly demanding workout may stay slightly elevated the next day also.

Remember exercise does "fire up" your metabolism, however, I would not lessen the exercise quantity permanantly, perhaps have a lighter week every three week. If this amount of exercise is new to you, your body has to make changes and accomodate the new demands. However, it does adapt.

Example - in 2005 (my first full year of cycling), 200km on the road per month was a big month. This year I have averaged 544km per month (for the first 4 months) on the road (not including spin and mountain bike).
My partner began in 2004 with 200-300km weeks on the road. At the moment he is base-training and putting in 600-800km weeks.

My point? I could not have handled the riding I am doing now in 2005 - I would definately have been overtrained.

KFergos... if you are coping just fine and just tired, I would not worry too much. With those distances, you should feel tired. As for amhenorheah (how DO you spell that word???) - many female athletes sacrifice their period. There may be nutrients you need to ensure you are taking, so do consult with a doctor. But based on what I know (which, I admit, is limited) I wouldn't be too concerned - particularly if the doctor isnt.