Hi!When I get that "so tired I feel sick" feeling, I end up having a bout of IBS-D. Sometimes I go too long and too hard and don't stop until I am done in. I have found that i need to get to bed for the night before I feel that way. Then I do better the next morning. Since I have been taking two Caltrate every day at lunch, I have felt 90% better. I still have to avoid that severe tiredness though. Hope this helps.wpmom