Why did my heart rate drop to 32 after a knee replacement surgery?

Hi doctor,

I was in the recovery room after undergoing total knee replacement (TKR), and my heart rate dropped to 38. I felt weak and tired, so the nurse added something to my IV, and I felt fine, and all vitals returned to normal. They took me to my room, and I ate and was resting. My heart rate dropped to 32 beats per minute, and I was nauseated, cold, sweating, and very pale. The emergency team was called, and after 20 minutes, they got all my vital signs back to normal. They wired me with a portable heart monitor to see what was going on. The next day, I was doing knee exercises with the nurse, we walked down the hall and returned to my room, and I was put in a chair to finish my exercises when I passed out. According to my heart monitor, my heart stopped for 30 seconds. I had no idea of what happened. After 30 minutes, the emergency team had all my vitals back to normal. I was taken to ICU. My heart chambers, valves, and arteries were normal. I am 61 years old with a history of stage IIIC colon cancer with six months of chemotherapy. I have no history of blackouts, heart issues, or family history of such. Next day they put in a pacemaker only as a precautionary measure, they cannot explain how or why it happened. Three weeks after, I do not have any problems. Do you think the TKR could have caused this to happen?