Sinus tachycardia of the heart at rest, why does this happen?

Some ideas: Pain or anxiety can cause mild tachycardia near 100. Possible hidden causes are anemia & overactive thyroid which are detected by blood tests. Sounds like your doctor already did an ekg, which would rule out an abnormal heart rhythm if its really >100. Heart & lung conditions (with low oxygen) can cause sinus tachycardia too but you probably would have other symptoms like short breath. Good luck.

Tachycardia: Sinus tachycardia at rest can have many causes including deconditioning, anemia, hyperthyroid, fever, infection, trauma, anxiety, drugs, smoking, pots, tachy/brady syndrome, arrhythmic sinus tachycardia, and more...