If the bowel movement is hard and you are straining, that could cause some of the symptoms. Eating more fiber, drinking more fluids and a stool softener may help.

If constipation is not a factor, then you may have irritable bowel syndrome especially if you are having abdominal cramping right before the bowel movement and if you have alternating diarrhea and constipation.

Another possible reason could be an imbalance in your sympathetic or parasympathetic nervous systems. This could be caused by side effects from medications especially blood pressure medications.

The symptom of rapid heart rate associated with dizziness and lightheadedness strongly suggests a rhythm disturbance in the heart such as atrial fibrillation, supraventricular or ventricular tachycardia.
A 24 hour Holter monitor can record every heart beat for 24 hours and then your doctor can see exactly what is happening.
Some rhythm disturbances in the heart can be very dangerous so you should see your doctor soon and ask for a Holter Monitor.Some medication may be prescribed which can control it.

Please acdept answer if it has ben helpful

Thanks
Dr. F.