Why would a heart rhythm tend to beat irregular &/or v fast (very briefly) when you lie back down (after standing up for a few mins, from lying down for quite a while), i cannot figure out the logic of this whatsoever & dr google has not been helpful?

Orthostatic hypotension is most likely the cause. Sudden change in BP can cause to autonomic nervous system to kick in. This controls heart rate, breathing, and metabolism. Hope this makes sense: ref: https://www.sciencedaily.com/releases/2018/03/180328092517.htm ref: https://my.clevelandclinic.org/health/diseases/9385-low-blood-pressure-orthostatic-hypotension

Dr. Google is rarely helpful and often not-healthful. I recommend spending the money it costs to subscribe to uptodate.com for a month and see how you like it. If you do a years subscription is less that $1 per day and will save you time, money and maybe even you life or that of loved ones.