LSD activity builds thicker heart muscles, but that muscle is weak.
Heavy strength training builds stronger heart muscle without much hypertrophy.
The size of one's heart is not important when compared to the function of one's heart.
Your attempt to drop weight quickly most likely significantly negatively affected your mineral levels (aka electrolytes). The heart is a biochemical electrical machine, thus minerals are heavily implicated in rhythm disturbances, especially with those who try to sweat out bodyweight without replacing electrolytes that get depleted.
Update on the relationship between magnesium and exercise.
Magnesium is involved in numerous processes that affect muscle function including oxygen uptake, energy production and electrolyte balance. Thus, the relationship between magnesium status and exercise has received significant research attention. This research has shown that exercise induces a redistribution of magnesium in the body to accommodate metabolic needs. There is evidence that marginal magnesium deficiency impairs exercise performance and amplifies the negative consequences of strenuous exercise (e.g., oxidative stress). Strenuous exercise apparently increases urinary and sweat losses that may increase magnesium requirements by 10-20%. Based on dietary surveys and recent human experiments, a magnesium intake less than 260 mg/day for male and 220 mg/day for female athletes may result in a magnesium-deficient status. Recent surveys also indicate that a significant number of individuals routinely have magnesium intakes that may result in a deficient status. Athletes participating in sports requiring weight control (e.g., wrestling, gymnastics) are apparently especially vulnerable to an inadequate magnesium status. Magnesium supplementation or increased dietary intake of magnesium will have beneficial effects on exercise performance in magnesium-deficient individuals. Magnesium supplementation of physically active individuals with adequate magnesium status has not been shown to enhance physical performance. An activity-linked RNI or RDA based on long-term balance data from well-controlled human experiments should be determined so that physically active individuals can ascertain whether they have a magnesium intake that may affect their performance or enhance their risk to adverse health consequences (e.g., immunosuppression, oxidative damage, arrhythmias).
I would suggest looking into these:
Particularly their "sports" version. If you aren't able to purchase them (they might only sell to practitioners) and you want them, PM me. Natural Calm plus Calcium would be a lesser option (it contains Ca, Mg, and K) in this situation. If it were me, I'd be supplementing my water all day long until the issue resolved.
I can only hope you learned from this experience.