I also had the exact same reaction. I had a complete physical in April 2021, before changing jobs and all blood work was in normal range. I had the 1st dose of the Pfizer vaccine in August and the 2nd on Sept 4. I started noticing fatigue, as it progressed by heart rate was elevated. I went to urgent care and they did an ekg, chest x-ray and blood work. My hemoglobin dropped to 6.8. They also suspected an internal bleed. I went to the emergency room and was given 2 pints of blood. I have never has an issue with anemia before. Nothing in my normal routine has changed. I can only assume it is from the vaccine as well.