I'm 18 and have been working on losing weight for about 10 months. I am an endomorph and eat as I should for my particular body type. For the first 2 months I would walk for 30 minutes every day, then I started walking faster and for 45 minutes, even running for short distances to jump my heart rate ever so often. On the 5th month, I decided I'd start really doing intervals. I strapped a heart rate on me and started working. When I first started, it only took 30 seconds of running for my heart rate to reach over 210, and 30 seconds was as long as I could last, for I'd get dizzy, nauseous, and my legs would like buckle under me. It'd take at least 5 minutes for me to even begin to recover. It's been 5 more months now and though I've been boosting up the periods and distance of my running, I can still only run for about 2 minutes before I feel the need to faint. My heart rate is still getting over 200. I don't really know if this is normal or not, but it feels wrong. I've gone to a doctor and was told I didn't have asthma and that everything was fine, but I still can't shake the feeling that something is not right. Any ideas? Also, my starting weight was 180 and my current is 155. My mother and her dad have also had this same problem since they were younger, even when they were at their peak physical condition, so I think it might be hereditary or something but no one knows what it could be.