My stomach sticks out quite a lot?

I don't know what it is because it's not like it's flabby - it's hard, you could punch it and it wouldn't hurt me. My teacher said it's my singing muscles kicking in now, but how do I make it flatter? It isn't fat, because I work out and everything. It's just like, a sticky out bit. It's the lower half of my stomach, the upper half is really well toned. Anyone got any exercises for the bottom of your stomach, or else what the hell is going on with it? :/