My puppy has a big hard knot on her lower belly! she is a 5 month old lab/golden retriever!?

take it to the vet!!!

I also have a golden retriever but he is 6 going on 7 years old and also has knot but on his chest. We took him took the vet but she said it did not seem like anything at the time being, she just told us to keep track of how big the lump got so just measure it every day. My suggestion for you is to take your puppy to the vet to check it out it could be something or nothing, but take her to vet to check it out. I would start taking measurements of the knot daily so when you go to the vet you can say how big it was on Nov__ and Nov ___... Better safe than sorry! I Hope she is okay!