With all due respect, this is not just entirely incorrect, it's dangerous advice.
First of all, if you have a newborn baby with a heart rate of 220 bpm you should call an ambulance - immediately. Healthy babies are not born with a heart rate of 220 bpm. You are confusing maximum heart rate, the rate your heart can safely beat under maximum exertion, with normal heart rate. Unless a newborn is crossing the line of the London marathon, 220 is not normal.
Secondly, the OP was assessed during pregnancy when a number of things can push heart rate up. She's no longer pregnant, therefore these factors do not stand. Additionally she has a family history of severe cardiac issues. She warrants reassessment.
Thirdly, the most important thing here is that the OP is symptomatic. If your heart rate is high and you feel wonderful, then yes, that's likely just your 'normal' but if you are fatigued, out of breath, feel like your body is struggling, then something is amiss.
Can I suggest you edit or remove your comment. It's not ok to advise strangers on the internet to ignore cardiac symptoms and I would hate to think that in the future a new mum trawling the internet for advice on her newborn's racing heart would come across your comment and mistakenly believe they have nothing to be concerned about.