First off, you do really need to follow through with your doctor's instructions. Prozac and Zoloft are both SSRIs, but they aren't the same drug. It takes about 2-3 weeks before Zoloft is at an adequate level in your system to prevent depression, and between 2-3 weeks for Prozac to get out of your system. The overlap period your doctor prescribed was designed to help keep you from experiencing any intermittent issues. That said, shakiness and feeling hot are possible side effects, but may also be symptoms of drug interactions (Don't combine Zoloft with Advil, Motrin, or Aleve). You should contact your psychiatrist immediately to inform him or her about your new symptoms, as well as not following the planned transition.