Have you had your RBC magnesium levels checked? It may be nothing, but the heart likes a steady supply of magnesium, and some/many people just don't get enough...depending on where they get leafy greens, how much they eat of it, and if it's replete with magnesium due to soil enrichment (soils these days have been badly depleted in commercial contexts). If you're not sure, I would recommend you start with a daily dose of about 200 mg of Magnesium Citrate (the salt that is the most easily taken up by the human digestive system, although there are several other salt varieties of magnesium).
Please do run it past your GP or other professional care-giver before you modify your diet. If it turns out to be the case, it's easily and quickly corrected. But check first!