Hi,
I'm sorry for all of your recent losses. That must be hard.
I noticed that you mentioned rocking your son. I know that many, many, women begin having panic attacks at some point after giving birth. That's when mine started, a month or so after giving birth. Sometimes it's as simple as hormones that starts it. The stress you're feeling now may be contributing. When I had my second child, there was a problem and even though I was fine, I was put on a heart monitor. After that birth, I was paying too much attention to what my heart was doing for a while, convinced it would just stop. It didn't. I'm still here.
But that fixation faded away on its own, and as my life got busier, I was fine and didn't have anxiety issues for several years later when in another stressful situation.
Everyone's heart does those funny beats.. happens all the time. If your doc says you're fine, then you're fine. Our hearts use those funny beats to regulate the rhythm, if we have too much caffeine, and especially if we don't get enough sleep. My stepdaughter had those funny beats for 2 weeks straight when she gave up caffeine, and her doc said she's fine. It's quite possible something hormonal, or just being overly tired, made the attack happen, and the recent events probably gave your mind something to work with thinking that your heart is bad.
My best, unprofessional advice, is to get out more, take more walks, and live your life as fully as possible. It's not the panic attacks that mess with our lives, it's the fear of them.