low heartrate upon standing

A little over a month ago I went to the hospital and was dx with low potassium.  I was given an ekg which was fine and blood test which included the test for heart attack.  Everything was fine.  A week later I had a panic attack Im guessing and went back to the er done same tests and everything was fine again.  I was given ativan for anxiety and sent home.  Now my heart rate drops upon standing up for a few seconds then returns back to a normal rate.  I dont feel dizzy when this happens its just a strange feeling.  Lately though I have been having stomach problems with bloating, burping and pain every now and then in my upper left abdomen I dont know if this is related to it or what.  Please help because Im sick of panicing over this.