Her face looks different depending on what day it is -- in some performances, her nose looks larger, other times it doesn't...it's the same with music videos, too.
I think it can be attributed to makeup (during live performances/appearances), and also computer editing (in music videos....I think it's especially clear in certain parts of 'Bad Romance').
If you look at this video, where she doesn't have much makeup, I think you can see her 'real' face pretty clearly.