I wasn't present during the shot, and I can't comment exactly on what the popping sound was.
Typically a new syringe and needle is used for each tetanus shot. Sometimes, they are equipped with safety devices, which may be the source of the extra sounds that you heard.
Different clinics use varying needle systems that can cause a variety of noises during the shot.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.