After coming home from a normal lunch at a local restaurant , I was visiting outside with our neighbors on their patio. We visited for maybe 20 mins, and as I was walking across the yard, I began to shake uncontrollably all over including arms, legs, feet, head,; followed by extreme nausea and a feeling of drowsiness. My heart felt like it was jumping out of my chest and felt heavy. I felt extremely tired, and felt like my eyes could roll back into my head. It lasted 40 minutes, my BP was normal, 120/68. Afterwards I took a three hour nap. Is this a one time thing or should I be concerned??