Jen is having something called sinus tachycardia.
Sinus tachycardia (also colloquially known as sinus tach or sinus tachy) is a sinus rhythm with an elevated rate of impulses, defined as a rate greater than 100 beats/min (bpm) in an average adult. The normal resting heart rate in the average adult ranges from 60–100 beats/min.
Being caused by dehydration caused by infection and fever. They are hydrating her with more fluids.