I had a similar experience with my metabolism, Jess42, and I'm not sure if it's just the way my body is or that I trained it to be that way like an idiot. As a kid, I was doing cardio literally from sunrise to sunset every day of the week (riding a bike). Sometimes I'd even ride my bike several miles to get to a track and run for an hour or so, then hop on that bike to ride home again. Add wrestling to that and omg. And I wondered why at over 25,000 kcals a day, I couldn't gain any weight.
I thought Parris Island was going to put me in the best shape of my life, but it wasn't as hard as wrestling practice was. I was on double portions with a waiver because my weight was too low and I actually started falling out of shape while I was there. Picked up the first 10lbs of weight gain during that time. Now at almost 40, it comes on so fast that I can't figure out how to get rid of it. I've even tried nutritionists to get the calories right because nothing seems to work. I'm starting to believe that the only way I'm going to lose weight is to go back to doing what I did as a kid and start training for marathons now.