Is a quick bodily shiver or tremor, often occurring only in the leg, accompanied by a sneeze, considered normal, and does it necessitate doubt concerning ritual purity (tahara) or the ritual bath (ghusl)?

We advise you to turn away from whisperings (waswas) and not pay attention to them. If you feel a tremor or anything that follows it, do not inspect or look to see if anything has exited from you. Instead, build upon the fundamental principle that nothing has exited until you attain certain and decisive conviction that something has. Do not perform wudu' or ghusl (definition) merely due to doubts. You may consult doctors or the consultation section of the website regarding what you are experiencing.