Heart palpitations haven't been a major problem for me, but I have had them on occasion since having the Mirena out (not while it was in). It's always been during period/ovulation, as I recall. I remember one morning waking up feeling my heart skipping beats and doing strange things. Scared me but was temporary. I have also had a lot of racing heart too, which is unusual for me. Make sure your magnesium is magnesium malate or citrate (avoid oxide).
It is definitely not unusual to have new symptoms develop after the IUD is out.