What causes a rapid heartbeat even when I'm feeling relaxed?

Several things. Being not relaxed is just one of many things that affects heart rate and rhythm. You could have an overactive thyroid for instance. Fever can make you heart beat faster or being dehydrated. Some times the heart rhythm can just become abnormal such as when a person gets atrial fibrillation.
Tachycardia. Anemia, hyperthyroid, small heart, days autonomic, weak left ventricle, valve disease, aneurysm and more including normal for you.

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