Can constipation cause dizziness/lightheadedness. i usually am dizzy when unable to have bm. it is uaually followed by diarrhea. can constipation press on a vagus nerve?

Normally not: Anything is possible but your symptoms suggest that you at least need a GI workup. The vagus nerve is too far away to be directly affected. Are you anemic? If you are regualarly constipated are you sure that it is diarreah and not chyme(liquid processed food)pushing past old stool? Sometimes hypothyroidism can cause chronic constipation. Perhaps you should check with your primary care.