why is their tachycardia with hypotension in fever

The fever response triggers many communicating substances and some of those spped up the heart rate.  This allows more blood flow around the body and aids in the defense of the host.  Hypotension does not always accomapny a febrile response, in fact it is a rare event (when compared to the number of febrile episodes each year).  Fever with hypotension is a serious combination and warrants evaluation quickly by a physician, often in an Emergency Department.

Peace,

Daniel A.