Chest Heaviness, Breathlessness, Dizziness, Fainting. Sweating, Burping And Passing Gas. What Happened?

I was making breakfast and suddenly felt a heaviness of the chest, and felt the need to take deep breaths. Then I felt dizzy and fainted for about 30-40 seconds, during which my eyes rolled, according to my husband. When I regained consciousness , my heart beat rapidly for about 20 seconds, and I broke out in sweats, and felt very tired. For the rest of the day, I kept burping and farting nearly non-stop. This happened twice in 3 months. In both instances, it happened in the morning when I was on an empty stomach, and I was either menstruating or just finished menstruating a couple days ago. The doctors checked my BP, red/ white blood cell count, cholesterol & triglyceride level, thyroid, did CT scan, EEG , brain MRI, chest x-ray, ECG, Evoked Potential, even a spinal tap. All the test came back okay. I m very worried, please help! I am a 43 year old female. 5 3 , 115 lbs. Generally healthy, except for a slightly high cholesterol level (6.2), and my LDL is at borderline level. Triglyceride and blood glucose are both normal.