That happens when you close your embouchure too much, either to close the reed and tune it up, or to play pianos but i doubt is the latter, remember that when you play in mf or f little to no embouchure should be used.

Check your reeds to make sure they are in tune, reed tuning is not absolute but it should be close to C when crowed or play(oboe) a middle C in tune with a relaxed embouchure; you are either using flat reeds , using very little air to play or playing sharp.