Hi there,the only way to diagnose Cardiomyopathy is for your Daughter to see a Doctor and under go tests. If you believe her to suffer from this condition then please seek medical help asap. Tiredness and weakness after any exercise are some of the sysmptoms, also a history of heart problems in other family members, irregular heart beat is another. 

I have put this link in for you  http://www.childrenscardiomyopathy.org/site/description.php , however, once again I urge you to seek medical intervention to test your fears as this condition can be life threatening if untreated. Best wishes and hope you get your answers soon and the news is good.

Luv Winnie.xx