Could potassium level 3.5 be simple cause for my irregular heart beat, muscle cramps, exhaustion? Blood labs, chest xray, echo all read normal.

Potassium of 3.5. Although your potassium (k) of 3.5 meq/l is low, it is normal by lab standards. You should also have your doctor check your magnesium (m) level. A low m level can cause a low k, which cannot be corrected until the m level is normal as well. If both your k and m are not low on another blood test, you need to see a cardiologist to determine the cause of your cardiac problem. Good luck.