Could combining Letrox, magnesium, and B vitamins have caused heart palpitations and malaise?

This is not a typical reaction to the use of such a combination. It may be worth introducing these medications gradually. First regularly Letrox, then after a few days add D3 and after a few days B vitamins and observe the body's reaction.

If you notice any side effects, you should report them to your doctor, who will decide what to do next.
You can find out why you should report adverse drug reactions here:

https://www.gdziepolek.pl/artykuly/jak-i-dlaczego-zglaszac-dzialania-niepozadane

If you do not have the opportunity to consult your doctor, you can use the teleconsultation here:

https://www.gdziepolek.pl/telekonsultacja

The advice is held in Polish.

Here you can compare the prices of magnesium and B vitamins in online pharmacies and order with home delivery (also abroad) or pickup at the pharmacy:

https://www.gdziepolek.pl/produkty/45778/magne-b6-forte-tabletki-powlekane/apteki

https://www.gdziepolek.pl/produkty/45771/maglek-b6-tabletki/apteki

https://www.gdziepolek.pl/produkty/83177/vitaminum-b-compositum-polfa-drazetki/apteki

In cities such as Warsaw, Wroclaw, Łódź, Krakow, Katowice, Lublin, Poznan or Bialystok, products can be ordered with instant delivery even in 1 hour thanks to the Wolt Drive service - check if the service is available in your location: https://www.gdziepolek.pl/koszyk/szybka-dostawa?src=zf