Sudden feel heart sinks and stops. This?

My age is 24, (No addiction of alcohol and smoking) One day I was going to another city to give an exam, so I was walking  and going to the train station. As I reached the station, suddenly I felt a shock in my chest and it felt as if my heart was stopping but there was no pain,  my hands and legs gets  started trembling, but after some time it became normal, then after a month I was sitting free and suddenly this thing happened, before this all I also having some other symptoms like sudden feel choking during eating food , breathing problem in deep breath but this will happen . I had ECG, 2D ECHO, ULTRASOUND , CHEST XRAY, CBC, LFT, RFT, THYROID, LIPID PROFILE, ELECTROLYTES TEST, ENDOSCOPY, HBSAG , HIV, HEPATITIS (B and C) all are normal, I consult with 2 CARDIOLOGIST (MBBS MD DM), ENT SURGEON ( MBBS MS) , 2 PULMONOLOGIST ( MBBS MD) , UROLOGIST ( MBBS MS) they says nothing to worry all are normal ,  then why did this happen to me, please reply what  actually is this?