No, every atom in your body was not present at the time of the Big Bang. The Big Bang is the scientific theory that describes the origin and early development of the universe. It occurred approximately 13.8 billion years ago. At that time, the universe was extremely hot and dense, and only simple subatomic particles like protons, neutrons, and electrons existed.
Over time, through processes like stellar nucleosynthesis and fusion in stars, these basic particles combined to form more complex atoms like hydrogen, helium, and trace amounts of lithium. These atoms later participated in the formation of galaxies, stars, and planetary systems.
The atoms in your body, such as carbon, oxygen, nitrogen, and so on, were created through the life cycles of stars. These atoms were not present at the time of the Big Bang but formed much later in the universe's history. They were synthesized in stars through nuclear reactions and were eventually incorporated into new generations of stars, planetary systems, and, ultimately, living organisms like yourself.