Severe heart pain for3days, tachycardia, short of breath, lightheaded, explosive watery diarrhea, stabbing back pain after givirus. Why? See dr or er?

Diarrhea. From the description of your symptoms of lightheadness and tachycardia, you are dehydrated. Get to an er for IV fluids to replete your intravascular volume and find out why you have your diarrhea. Please, do not wait.