Negative space - UV mapping


I am modelling a character for the first time. I have created seams on it and I have done unwrapping on it. But when I try to do uv unwrap again, it says "object has negative scale, unwrap will operate on a non-flipped version of the mesh " and the result is incorrect uv mapping. The result is incorrect uv map islands in uv view. How to fix this? Thanks.

does it have something to do with mirror modifier?

Ok, I have done my character with mirror modifier and done the seams on it as well, uv unwrap works correctly. But when I aplly mirror modifier on the character, and then continue adding seams and redo uv unwrap, then it gives that error message about negative scale and thus uv comes out incorrectly in uv map box. But how do I fix that?