Why does my body do this when ever i start working out, and when i don't?

i have been on the wrestling team, and i worked out a lot, but whenever i'm on a schedule where i cannot workout, I turn straight up fat. but for example, i've been doing a dance Camp for my school, and i don't feel like i've been working out THAT much, but whenever i do physical activity for a few days, i look like a freaking supermodel, a six pack, everything, (i genuinely don't mean to brag) but it's shocking how fast it happens. why is this? however, I feel I cannot gain much muscle mass, no matter how hard i work out, at least more than i already have.