A hard thump to the chest does deliver a small amount of energy capable of capturing the heart.
It can be fatal sure.. In order for this to happen the thump would have to occur at a very precise time in the conduction cycle..
Your conduction system exists in this time window for about 5%-8% of of any given unit of time..
Furthermore the impact would have to be sufficiently forceful, yet meek enough not to cause any other trauma (ie broken ribs). Just simply sustaining (as it appears you have) such a precise impact at all is a feat!Additionaly this has to occur at a period in which conditions were favorable to capture, regardless of whether or not the timing is lethal.
Realistically these favorable conditions are only present for about 4 - 6 hours per day.
For an impact to be sufficiently hard, ocfur at the right time of day and to make impact within the lethal window is rare... Unbelievably so..
To be precise: The chance of an otherwise non lethal/non traumatic impact directly to the sternum actually killing you is about 1 in 4500. This doesn't even account for the probability of that "perfect" amount of force hitting you in the first place!
If every hit were to theoretically be "perfect" the odds are still roughly comparable to winning $1000 from a State Lottery Scratcher in most states...
Of course; If this were to happen you would die instantly.. Not days later. So no, I don't think this is your problem.. I am 100% certain you wouldn't be telling us about it here if this were the case.
In fact we'd probably be hearing about it on the news or social media or something as one of those "Strange Stories" kind of things..