I play picc at my school. But i'm also taking private lesson for flute outside of school. I find that after i play the picc and switch to the flute, my tone and embouchure is gone. My flute is very airy after i play picc. I dont' know what to do about this. I ran into a problem last week where i had to play picc at school and had flute music fest the next day. I had to stop picc for a week prior just to work on my flute playing. Any suggestions? Plus the current flute i'm using is a Muramatsu DS standard.

Just practice. It's only a matter of time for you to get accostumed to the change.
In general, the piccolo embouchure is smaller than flute. As you mentioned only a problem when switching from picc to flute, I'd advise you to try relaxing exercises before playing the flute again. Do it over and over and it will come eventually.

just relax your embrochure before playing, and practice the switch. try blowing out your lips like babies do, it works well for me.

As others have said, practice switching back and forth. It will get easier. When you go back to the flute, start with some long tones to focus your tone and embouchure.

what kind of relaxing excercises though?

relaxing exercises like...
blowing your lips out like a baby. =)
Also, when you go back to your flute after playing pick (the first few times you practice it) try doing the "smile" thing your elementary band teacher taught you.
It makes your embrochure much more focused on flute when you smile while playing (at least when your normal embrochure is out of whack). JUST DON'T SMILE ON PICC THOUGH! Instead, pretend to slightly whistle into the mouthpiece...you probably already knew that though. *smacks head* Man I feel stupid today. Must be this cold...