hi..thanks for the query. May I know what is your age & whether you are on any long term medications or received a short course of medications recently? Assuming you have neither of these, first thing we need is to conclusively rule out a condition named arrhythmia which occurs due to dysregulated firing of hearts rhythm generator or pace maker resulting in symptoms of sudden onset pounding heart, lightheadedness or dizziness. This can be ruled out by a ecg. Once arrhythmia is ruled out; what we are seeing is essentially anxiety either situational or occurring suddenly. This can be easily taken care of by giving sos clonazepam/etizolam (benzodizepines) and depending upon cause of your anxiety SSRI group drug like sertraline, escitalram which will take care of your anxiety symptoms altogether. Hence dont worry, consult your primary care doctor & I am sure you will be fine in no time!! good luck