Sometimes I will just be sitting down or lying around and all of a sudden my heart will just pound in my chest so hard for at most 15 seconds. Sometimes it is a bit painful and sometimes I get a little shortness of breath too. I am not stressed out and I sleep plenty and I am not anemic. Can anyone please let me know if they know what is happening to me?? It isn't that often I have felt it about 4 times in the past 2-3 months.