Cut back 600 calories a day (same high-fiber nutritious foods, just smaller portions) to lose weight. still daily bms but now hard. diet the reason?

Very possibly: ....but I would think the bigger problem might be water intake, though that surprises me. Your question suggests you are knowledgable about healthy eating and I would guess that you are pretty good about drinking water. In any event, especially with this few calories you need 8-12 glasses a day. Make sure you are getting at least 1000 ca/day; fewer is unsafe. Best wishes:)