The palpitations are usually the sensation the patient feels through the chest wall, sometimes up into the neck. They suggest an arrythmia or maybe just a tachycardia (heart speeds up spontaneously for no apparent reason, but the runs usually limit themselves to just a few beats, six, ten...). Often the arrhythmia, if that is what it is, will be a PAC, or a premature atrial complex. The atrium has extra beats when they follow a paused beat, and the result can be a substantial thump. These, too, can happen several times in succession and then go away on their own for days.

Your new drug is a 'rate control' medicine meant to keep a lid on your heart's rate when it does go into arrythmia or into tachycardia...I don't know which it is. The idea is to not allow the heart's rate to rise much above 100 BPM for long. If you have a pulse monitor, and you feel the dysrhythmia happening, keep monitoring to see the rate, say once an hour. If it stays above 100 for 24 hours, you should go to the ER or call your cardiologist if you know he/she'll take the call or get back to you inside of a couple of hours.

The enlargement of (so far) only the left atrium tells me, a non-expert, that you have some atrial arrhythmia and it has gone on long enough that your atrium walls have expanded due to the strain on them. The walls have stretched IOW. There are degrees to the enlargement. Hopefully, you don't have much of it. I won't go into more details because they can weigh on the patient and ramp up their anxiety.