I Am A Female 54 Years Old Who Does Not

i am a female 54 years old who does not smoke. i had ASD for which i had open heart surgery in 1994. i also have SVT i take acebutolol once a day for that. i've had no problems until the last month. I had not eaten or drank much water that day, but had a few drinks. later that night i fainted going to the bathroom and really hurt the right side of my body. i'm recuperating but ever since then, my heart is beating irregulary. I went to doctor and he said every sounded fine and took blood work that showed i had low magnesium. I got the supplements right away but it's still doing it a lot during the day but not at night when i sleep. it's driving me crazy. any advice please?