From your description, it sounds like a skeletal little muscle fasciculation or twitch (there's a video of a fasciculation on youtube). You can google-image 'human chest anatomy' to see that there's nothing heart-related up where you felt the sensation.
You mention that you aware that you suffer from anxiety. May I ask if you are under treatment for that?
Thanks for your response. The simple answer is I have anxiety that causes to me to race to the ER sure i'm having a heart attack. It's awful to say the least. It is usually very controlled but once every couple of months an attack happens that I don't see coming which winds up in all these tests. The palpitation if that's what is was was so high up I thought there was no way it was a palpitation. It almost felt like it was right under my skin.
I will attempt to give an anwer but all I can say is "it could have been" Sometimes I feel my palps in my chest but sometimes I feel them in my stomach.I also get really shakey and sometimes clamy. Might I ask why you had all that testing done? Some of the folks on here can give a better answer than me but it would help to understand more of what you have going on.