I've been experiencing palpitations and sudden quick shortness of breath...what could it be? I m a 23 year old male with no past medical/surgical history. I do not take any prescription drugs; however, for the past 4-5 years, I ve been taking the over the counter weight loss supplement called Xenadrine. I took it for 3 years WITH the ingredient ephedra, and 2 years without it, after the FDA removed that ingredient. My weight went from 310 to 155, and recently up to 185 (i m trying to lose 30 pounds but that s a whole other story). For the past few months, I ve had a few very scary episodes of sudden shortness/loss of breath accompanied at the same time by a feeling in my chest as if my heart stopped for a split second. This does not happen when I m active - it ONLY occurs when I m at rest, such as when I m sitting or laying in bed. There are no other symptoms, and it only happens about once or twice a week. What could this possibly be? I prefer answers only from healthcare professionals please, because I am not interested in uneducated guesses. Thanks.