Pit Bull died suddenly.

My Female Pit bull is about a year and half to 2 years we got her from a neighbor that was Moving out she was a wonderful dog she also got pregnant couple months after we got her with my uncles dog that we were watching for a couple months but didn't know that she wasn't spayed. Well happy to say she had 5 Beautiful Puppy's 4 Males and 1 Female. We kept the female and raised her with the mother. Its been about 4 months since the birth she's gotten her size back and looking Really healthy until Today Mar. 29, 2011. I woke up this morning she was playful and needed to go outside i let her and her daughter out side. i was in side for about 5-10 minutes. Then i started hearing Really Loud Whimpering out Back. As i came out back i noticed her lying down on her side Whimpering loudly still and her Daughter Licking her. As i came to about 10 Feet From her She Stopped and Became Lifeless. I put my hand on her chest to see if she was still breathing and feel a heart beat but neither were there. I was Baffled so I Began Giving her CPR without mouth to mouth just Compressions on the chest to see if that would Jolt her Heart or get a Reaction. After about 3 Minutes of that i received No indications that she was Going to Come back. I just would like to know what you think would cause this Sudden Death?? I would really like an answer if possible i don't mind the wait.