Cubase 5 Export problem


I use Cubase 5 and now I have a problem. At the beginning when I exporting music is heard equally in Cubase as the mp3 player. Now, my music which is mixed and is exported in mp3, increase the low frequency and distort the whole song and I do not know why. Please help me because quite a few projects stand for this because I can not get this out of what I have done.


And one more thing. If I Export music and import back in cubase, music play normaly why? I think there should be a problem with my sound settings.