My 3 yr old has pneumonia in the left lung and is having shaking chills accompanied by confusion (new symptoms) tonight. Should I go to the er?

Depends on situation. You have listed a DX & i assume the child was seen & rx started. It will take 24-48hr for any abx to catch up with the germ & make headway on the infection. If the kid is able to drink enough fluids to pee every 6 hrs & have some playful moments, you can wait a while to let the meds work. If the kidis sickly enough not to drink or otherwise work with you it would be time for a recheck.
Yes. Go the the er immediately. Confusion in the setting of pneumonia is a bad prognostic sign. Good luck!