Strange behavior in nursing dog?

So my 3 year old dog recently gave birth to 8 puppies about 4.5 weeks ago. Yes, I know, before anyone comes after me for not getting her spaid just know we had an appointment with the vet and then she got her period. Anyway, she's been fine and starting to wean them. Lately, she's been panting extremely... show more So my 3 year old dog recently gave birth to 8 puppies about 4.5 weeks ago. Yes, I know, before anyone comes after me for not getting her spaid just know we had an appointment with the vet and then she got her period. Anyway, she's been fine and starting to wean them. Lately, she's been panting extremely heavily and breathing hard. This occurred first when she went into labor for the first time. Today was the first hot day, but even on the cold days she did this. It might be because she's losing energy since her pups are very greedy and suck all the milk out of her, but if you know why she's acting like this please feel free to explain.
5 answers 5