This is the biggest questionmark unless the whole movie takes place in Cobb's limbo, in which case he could already be in a coma and beyond help in reality and Mal is trying to snap him out of it. This could explain why she doesn't appear straght away if she has to track him down in Limbo. It doesn't explain why she acts a bit crazy though so I'm more inclined to think she is a representative of his own subconscious trying to wake him up because the real Mal can't get deep enough.
Mal tells Cobb to stay
in limbo so she's definitely a projection of his subconscious. The movie's never completely clear on the consequences of staying in limbo too long. It seems to be it's less a danger to your physical health (you're just sleeping after all) as it is to your mental health.