After 4 years Dog fighting starts out of nowhere

My 6 year old Shih Tzu girl all over sudden started to fight my 4 year old Shih Tzu girl. All this time they were friends, got along great. Even had a litter at the same time and nursed each others babies early last year.
I don't know what happened. We didn't get new pets or moved. lt started about a month ago.
As soon she sees the younger one she goes after her, circles her, growls at her and then jumps on top of her and bites her. She won't let go until l separete them,
None is sick or pregnant or hurt, eat well and they play with other dogs in my house and even cuddle with the cat. At first the younger one would just stay still and let her go at it but now after a month she growls back at her, jumps at her but does NOT bit. And now, the younger one hides while the older still hunts her down, won't leave her alone.
Any idea how l can make this stop. Giving away my dog is not an option. We had them all from 5/6 weeks of age.
l treat them the same and don't know what happened a month ago. So far they still share the food but nothing else.
Thank you for any advice anyone can give me.

The best advice I have to offer you:

If you love your dogs...
If you consider yourself to be a responsible pet owner...
If you consider yourself a champion of animals everywhere...

SPAY AND NEUTER YOUR ANIMALS


:thankyou:

We had them all from 5/6 weeks of age.

If this really was the age that you brought these dogs home, it was far too young. Puppies NEED their mother until at LEAST eight weeks (I prefer 10-12 before seperation from the litter and dam) because they are constantly learning vital socialization skills and general dog behavior around the 4-8 week mark.

My first question is what kind of training have you done with your dogs?

Do you spend time individually walking or investing training into both dogs seperatley?

There are quite a few factors that could be at play here - have both your females been health checked recently?

When was the litter of the youngest female and how many has she had? (I agree with Mx3, spaying is incredibly important.)