two dogs not getting along

Question
I have a 7 year old Lab and a 7 month old mix breed.  The two were getting along great until about 1 week ago.  The lab cornered the puppy and was growling and barking at her. And yesterday I went to let the puppy in from outside and as soon as the puppy came in the lab completely attacked her.  I don't know what to do.  Why is the lab acting like this all of a sudden?

Answer
Many 7 year old dogs want nothing to with puppies and their sharp little teeth.  The mix would better have stayed with its mother and litter mates until now, learning when another dog says enough, it is enough.  

You are going to have to teach the dogs to leave each other alone.  When you are watching it, immediately correct it as soon as it goes for anything except its own toys.  In a quiet, but firm voice give it an ''Ah, ah, ah!''.  Gently remove what ever and replace it with one of her toys, or if older, hold eye contact until the puppy drops it.

You may need to crate the puppy when you aren't around.  Other dogs may
not be as bad as the young Labs I am plagued with.  Still your house and dog
will be much safer with the dog in a crate when you are away.  The dog may be
happier in its den than loose in the house.  It relaxes, it feels safe in its
den.  It rests, the body slows down reducing the need for water and relieving
its self.  Dogs that have been crated all along do very well.  Many of them
will rest in their crates even when the door is open.  I think the plastic
ones give the dog more of a safe, enclosed den feeling.  They are harder for
dogs to open too.    Metal ones can be put in a corner or covered with
something the dog can't pull in and chew.  Select a crate just big enough for the full grown dog to stretch out in.

Leave it some toys.  Perhaps a Kong filled with peanut butter.  Don't leave
anything in the crate the dog might chew up.  It will do fine without even any
bedding.  You will come home to a safe dog and a house you can enjoy.