I have a 5 month old cavalier king charles and everytime i take her for a run or to play she runs well in the garden (as far as running down the street in the morning she is out of breath in half a mile.
Every time we get back home she falls asleep for ages, is this normal??
She is still a baby, as Gustav pointed out, so she's not going to have the stamina of a mature dog. Plus, growing puppies need lots of sleep. Be sure to give her a rest period after she eats too.