There are many possible causes for bradycardia, and some of them are specific to someone undergoing surgery in the operating room.
Some people will have an intrinsic reason for bradycardia - such as a history of heart disease or arrhythmias- which predispose their heart to vary in rate/rhythm. However, given your young age and lack of any prior heart condition, this is less likely of a scenario. Also, the anesthesiologist who gave you the atropine would look for any abnormal signal in your EKG during the episode of bardycardia...any abnormality should be fully worked-up and evaluated with subsequent blood tests and EKGs after the surgery.
If, on the other hand, the bradycardia was simply a slowed heart rate, without any other concerning findings, there can be a very explainable reason for the slowing of your heart rate. When giving general anesthesia during an operation, there is a fine balance between giving enough medication so that the patient is comfortable and completely anesthetized, while not giving too much as to cause significant problems (such as death, for example). Each patient's titration is a little different and that is why there is so much monitoring involved during these procedures - to keep a very close eye on all vital signs to make sure everything is in check. So, sometimes, a dose of anesthetic can cause a change in heart rate or blood pressure, which is then treated accordingly with the appropriate drug...in your case, that was atropine. These adjustments in medication to treat changes in the vital signs are very commonly done and if there was a sudden, inexplainable reason for your bradycardia, there would have been a very thorough cardiac workup, like mentioned above. In addition, the positive pressure in the thoracic cavity from the ventilator that was breathing for you, can activate a specific nerve (the vagus nerve) that can cause a sudden drop in your heart rate or blood pressure...this can even be seen in awake patients who suddenly faint after straining heavily. This eventually recovers with supportive measures and time.
I hope this information helps. Overall, I would not be too worried unless the anesthesiologist saw something unusual. Feel free to let me know if you have any other questions or concerns. Whenever you get a chance, I would appreciate if you could rate my response. Thanks so much and take care!