I have a really sweet staffy x kelpie girl who is about 20 months. She has always worried me as she tires easily at the park. After a few ball throws or some running around with friends, she needs a lay down in the shade. Other kelpies I know can go all day, but today my girl lay down after about 15 mins of quite a bit running. It is winter where I live and quite cold today so I don't think she's overheating. We go to the park every day and she's walked again in the evening so I don't think she unfit, although recently she had an ear infection so we kept away from the park for a few weeks.
I thought it may have been a puppy thing but she is not really a puppy any more. I had a vet check her over and listen to her heart, and she said she was fine. She is not over weight. I'm wondering if this is normal or if I should get another opinion? Anyone else's staffy, kelpie or other pup do the same thing?