What mpc were you playing before when you didn't have this problem? It's not uncommon for there to be a period of adjustment when switching between pieces, especially if there are small, yet significant, differences in mouthpiece design. Have you discussed this with a teacher, because it could probably best be addressed by someone who could really see the way you are forming your embouchure and give you specific tips and exercises. That said, I know of experienced players who always seem to have a bit of leakage around the corners of the mouth; it's pretty much inaudible at anything other than a short distance away.