Have influenza. Now experiencing sharp brief breath taking all over chest pain and a constant dull ache over left side. Fast HR. ER?

How fast is "fast"? No fever? If you are an athlete with a baseline heart rate in the 40-50s (or not but in the 60-70s) and now you are pumping at a rate in the 90s or more, with a left-sided dull ache and painful breathing, then, yes. ER. They can evaluate you & rule out worst case scenarios. Make sure to tell them if you or anyone around you travelled or been sick. Hydrate aggressively on the way.