I received the Moderna COVID-19 booster on 1/13/2022. On the morning of 2/22/2022, I woke up lethargic and feeling unwell. I had exercised pretty strenuously the previous night, so I thought that might have been the cause. I worked all day on 2/22/2022. By the time I got finished working, I felt extremely tired and had a severe chill, and I couldn't warm up until I went to bed. When I woke up on 2/23/2022, I felt better than the day before. I worked all day again. By the time I finished work, though, I felt extremely tired again. The level of fatigue I experienced was unusual for me, because I usually have more energy, and I do exercises regularly. While I was in bed that night, I experienced extreme night sweats. I didn't have a fever, but I did have to change my shirt 5 times. On 2/24/2022, I woke up not feeling well, and I also couldn't breathe well. I have asthma, so I had to use my rescue inhaler frequently that day. By 2 PM, I had to message my manager to let them know that I was ill, and then I went to an urgent care clinic. When I got to the urgent care, my BP and temperature were taken. My BP was at a level that was higher than my usual, though it was still in the normal range. The doctor did an EKG. After seeing the results of the EKG, the doctor urged me to get to an ER as quickly as possible. At the ER, I was hooked up to another EKG, and I think that bloodwork was also done. I ended up lying on a bed in the ER for about 5 hours. They hooked me up to an IV, though I don't know what was in it. I ended up being admitted to the hospital. When I was in the hospital room, I was put on oxygen, as my breathing had gradually gotten worse. I don't remember many details, but I know that I had blood drawn several times on 2/25/2022. Various tests were run. The EKG results indicated that I had atrial fibrillation. Not only that, but I had a heart rate that was faster than normal. On 2/25/2022, they gave me diltiazem, metoprolol, and furosemide. And because they were giving furosemide, they also gave me potassium to take with my medications. I was also put on a blood thinner, whose name I don't recall. I ended up staying in the hospital until 3/2/2022. During my stay, I underwent various tests every day. The main reason why I was kept in the hospital for so long is because they wanted to ensure that my heart rate had slowed down to a safe pace before they released me. The doctors seemed genuinely puzzled by my condition, because I did not have a history of cardiac issues. They told me that the tests they were running included those for rare disorders, though I did not test positive for any of them. When they released me, they prescribed me diltiazem, furosemide, metoprolol, potassium, and Xarelto to take at home. My doctor decided that he wanted to do a cardioversion on me, and I underwent the cardioversion on 3/18/2022. Afterwards, I was back in my normal rhythm. I was taken off of furosemide, potassium, and diltiazem, but I continued to take metoprolol and Xarelto. On 6/16/2022, I underwent another EKG. On 6/22/2022, I had a follow-up visit with the cardiologist. At that point, he was puzzled, because my heart rate and rhythm were normal, and there were practically no signs that I had ever had atrial fibrillation or tachycardia. As a precautionary measure, he decided to have me continue taking metoprolol, though he did cut the dosage in half. He also had me discontinue the Xarelto. I was cleared for all of the activities I had engaged in prior to my having the atrial fibrillation. Ever since then, my situation has been normal. I've begun wearing a smart watch, so I can check my heart rate regularly. There have been a few instances that pointed to possible atrial fibrillation. In such cases, I would just rest for a while afterwards and recheck my watch; each time, my heart rate and rhythm were back to normal. My annual check-up is scheduled for the near future, so I will hopefully get a clean bill then. I don't know if these injections played a role in the symptoms I developed, but on 2/9/2022, I received vaccinations for tetanus and pneumonia.