Mommy Pit bull growled at me

My uncle owns a pit bull kennel where I spend a lot of my time. I have "adopted", so to speak, a female pit bull. She had her first litter and is about two and a half years old. She is the sweetest. She is also the reason why I spend a lot of my time at the kennel. I have trained her myself, but I tried picking up one of her puppies the other day, and something like a growl came out of her as she moved her head rapidly to block me from the puppy. I have raised her since she was a puppy and have never been afraid of her until then...is this a side effect of her recently giving birth? I really don't want to put her down, but if she is a hazard for my uncle and family, I will take appropriate actions, since she is unofficially mine. I guess I should be asking this to my uncle, but I don't want to let him know until I have your opinion because I do not want to get her into unneccessary trouble. Help!