Some people have fast metabolisms. What you could do is see your dotor and ask for a thyroid test. If your thyroid hormone comes back at a high level, then it means your thyroid is overactive. In that case, you can get some hormone therapy to correct the imbalance.
How old are you? If you're still going through puberty, this could be totally normal. Kids your height (males) sometimes eat a ton and don't gain any weight or see any real difference in how they'll look until after puberty hits. If this is the case, there may not be a lot to worry about, but still, you should see a pediatrician.
Ignore the GNC guy. They're usually idiots who have very little knowledge about fitness and nutrition. Tell this guy to come on bikeforums and justify his response to you here. We can debate where he's going oh so wrong with his response to you. I wouldn't give that type of response to you at all. I would tell you to continue biking but just see a nutritionist so that you can figure out how to eat healthy and have the energy to get through your workout. If you can see a registered dietitian, even better.
Please don't drink more soda. It's just high fructose syrup, and that's no good for you.
See a doctor, and get a referral for a dietitian or nutritionist. That's my thoughts. Never go back to that GNC.
Koffee