Actually, I sort of had a similar experience. I had heart racing before the zoloft if I would panic or something which was normal. But then once I started it I would just randomly wake up in the middle of the night with a racing heart or get it during the day without a trigger which never happened before the Zoloft.
I looked up the side effects of Zoloft and tachycardia or any arrhythmia for that matter is not listed. You may want to ask your doctor about this. It could be some anxiety but the Zoloft should be helping with that by now. Take care.