if you're gaining weight on the scale it's very unlikely that you're losing muscle. like jaime said, it's probably just under a growing layer of fat. so you're not losing muscle, just muscle definition. so I wouldn't worry about muscle wasting or anything like that...
also if you're doctor's not worried and you're still in a normal/healthy weight for your height and age, I wouldn't be too worried either. like AstroChick, I gained about 20 lbs between age 18 and 20, just as part of growing up. I went from 100lbs to 120lbs. it was a bit freaky at first but I eventually "grew into it" so to speak and though I've lost almost no weight now according to the scale (I'm 22, still around 120lbs) I have a lot more muscle definition than I did then (mostly from working out - I've gained a lot of muscle and lost some fat, so it evens out.)
so long as your doctor doesn't find any problems, your body probably knows what it's doing. just keep treating it well - eating well, exercising etc. - and you'll get along just fine