In "A Raisin In The Sun" Act 1 Scene 1 - Why did Walter ask Ruth what was wrong with her? (short answers)

Simply put, Walter does not know about Ruth's condition. 
The audience doesn't know until later, but Ruth is carrying the emotional and physical
weight of a pregnancy.  This is part of the reason why Walter asks what is wrong with
her. The other part of this is that there is the obvious tension in the opening of the
play regarding the arrival of the insurance check.  This causes a noticeable tension in
the air around the Younger home.  The fact that Walter is asking Ruth what is wrong with
her might also be reflective of Walter's own anxious state where he is more worried than
anyone else about that check because he has more riding on it than anyone else in terms
of his own hopes and dreams.