That's because there is something wrong with your practice method. Every time I feel unwell, I feel relaxed and full of energy. You may deliberately focus too much attention on the perfection of breathing and movements. It has the opposite effect. Remember to give priority to comfort and relaxation. And when practicing, it is best to get up in the morning and do only a few simple actions at night. I am a yoga instructor. I have been teaching for more than three years. Most beginners have this situation. Just stick to it and practice for a while. The main reason is that you are too nervous and always want to do well. Remember, no matter what you do, the first thing is to relax, meditate first, and then practice posture.