Thanks for your question.
There are a couple of possibilities to explain your symptoms which we call palpitations. You may be experiencing early heartbeats originating from the small chamber near the natural pacemaker (called PACs for premature atrial contractions), or from the ventricle) called PVCs for premature ventricular contractions. These early heartbeats leave a small pause until the next normal beat. Because the heart has more time to fill with blood, this next heart beat is much stronger and forceful and can be felt by most people. If you are having these premature beats very frequently, it can feel like your heart is pounding.
The other possibility is an abnormal cardiac rhythm that may be too fast. Though you say your pulse feels normal, not all of the beats may be strong enough to be felt by taking the pulse. These arrhythmias can be constant or intermittent.
It is possible that your discontinuation of the estrogen replacement therapy is a factor here, as there may be an excess of hormones being released from the pituitary gland called gonadotropins (FSH and LH). These hormones are suppressed by the estrogen. They often cause the usual symptoms of menopause such as hot flashes and sweating.
I suggest you make an appointment with your physician to have these symptoms checked out. He/she will ask further questions, examine you, and perform an electrocardiogram. Your doctor may perform some routine laboratory tests, so be sure to request thyroid functions studies as overactive thyroid is a common cause of PACs, PVCs, and cardiac arrhythmias. Then he/she will be in the best position to tell what is going on. If still a puzzle, further testing may be required.
Best wishes and feel better.
Martin Cane, M.D.