Check your Alignment! Always double check your alignment! When we put our flutes together, we are in a bit of a hurry at times....You should have a nail polish mark on your headjoint and body tenon to assure that your headjoint is aligned to your personal setting (somewhere close to the center of the embouchure blow hole aligned with the center of the c key on the body of the flute). Also, make sure the the foot joint is aligned with the center of the body keys lining up with the main rod of the foot joint. I have noticed that if my sound feels a bit off some days, I have missed this alignment by a tiny amount...proving that the smallest details add up to quite a bit.
So, take a moment to make sure you are aligned correctly at the headjoint and at the foot joint.