Spasm like sensations in left chest.

This has happened to me before Just happened again after months Its this odd sensation of my heart having a spasm(not a painful one by the way it feels like my heart beats irregularly for few seconds and chest a)when it happens i know its happening and then random spasms like weird beat...weird beat again two seconds to clear something i have had heart tests coming normal months ago.I have felt this same sensation in my right chest too once sometimes in my arm and i thought it was nerve related but it feels like my entire heart or just a small spot in my left chest.Also i may have POTS too cos just standing up or walking upstairs can make my HR jump to 150bpm and then go back down to 100bpm few seconds later. These spasm like sensations are scaring me(it doesn't feel like a flutter or a pvc it feels like My hr dropping to 50 and my heart spasming or few seconds i am scared i forget to touch my pulse while this happens)could this be muscle or nerve related?chest pain for few secs too