Heart Palpitations return

I experienced heart palpitations in November 07 over the course of 3 days. I went to see my doctor who had an EKG done which was normal (they saw the palpitations on the monitor and determined they were not worrisome). I also had a electrolyte panel and thyroid test done which came back normal. I had started 50mg of Zoloft 1 week prior so they had me stop and the palpitations disappeared over the next two days. I had taken Zoloft previously for 6 months (started in Feb and went off in July) and had no palpitations so when my anxiety returned in December, the doctor and i discussed going back on the Zoloft. I started 25 mg about 3 weeks ago and today the palpitations seem to have returned. Could this be a result of stress/anxiety (last week was so stressful my stomach was in knots and I left on vacation today) or the Zoloft? I know that the doctor told me 2 months ago that I was fine, but I wonder if should consider it the same? Any advice would be appreciated.