Actually, muscle mass -can- be gained rather quickly. The summer after I graduated, I got a set of dumbbells. Two 5, two 10, two 20 pounders. With just some basic exercises, three times a week, I bulked up pretty good. The only measurements I ever took were on my biceps, which went from 6" around to 12".

So yeah, I wouldn't put off the idea that you could be gaining muscle mass so quickly. :)