I fainted this morn due to pressurized bowel movements. Had exlax last night. Ekg, xray, urine sample are fine. Now feel dizzy and breathless. Why?

Vagus nerve. Fairly common reaction where bearing down during a bowel movement over stimulates a portion of your nervous system. This causes a change in your heart rate and blood pressure resulting in your symptoms. You should be sure to have plenty of fiber and water in your diet to ensure that your bowel movements pass without difficulty.
Fainting aka Syncope. It's not possible to give you a simple answer without a thorough evaluation. You may have experienced vasovagal syncope. The after effects may be from dehydration. In the end you should see a doctor possibly a cardiologist for further evaluation.
Get checked again. Did you go to a doctors office for the emergency room? If you went to a doctor's office, I would suggest going to an emergency room. The combination of fainting, dizziness, and shortness of breath are quite worrisome. I would get this checked right away.