I have had dark bloody stools for 3 days and suspected an ulcer which I have occasionally had. But today I am experiencing fatigue and dizziness.

Sounds serious. If you have an stomach ulcer and dark stools due to bleeding, it is a serious matter requiring emergency medical check up. Go see your doctor right away or go to a n ER to see a doctor to treat your bleeding ulcer. People can die from uncontrolled bleeding from a stomach ulcer...so do not be neglectful.