I am 20 years old and pretty healthy; no history of any heart or health problems. I am naturally physically fit even though I don't exercise much (about 5'9", 175 lbs, 10-12 percent bodyfat is what the Bodpod feeds out after 3 tests.) Whenever I begin to lift weights and/or do cardio, my heart feels like it's going to explode and I feel like I want to collapse and pass out, about 5 minutes I begin to start training seriously. If I sit down and rest for about 10 minutes, I start to feel okay again, but I am not tired at all and always feel like I have energy even though my body is rejecting it. Is this normal for someone who does not exercise regularly?