Why is my puppy so tired? Heart Condition?

I have a puppy which I think is a shepherd (or at least mixed with one). It is < 6 months old but gets very tired very quickly. After a few tosses of a ball she will be panting very hard and her heart is racing, so she lies down to recover. Even with a simple walk she has to lie down to take a break. I can tell she wants to be active but just dosn’t have the physical stamina to do it. This is not normal for such a young pup. I feel really bad for her.

What could it be? I was told by the vet that she may have a heart murmur. Would this cause it? If so will she grow out of it