Help, my older cat has turned violent to her younger playmate

We have 2 female cats. Abby is almost 4 years old and Sam is almost 2. Abby came to us after she was found in a field as a kitten still with her eyes closed. Samantha was adopted when she was 6 months old to save her from abandonment. They have co-existed together for 1 ½ years in a basically a happy relationship. Abby is normally a loving cat that at times is detached demanding affection at her whim while Sammy has always been loving and a bit skittish. Abby and Sam have always got along with the usually playful fighting. Abby loves to jump on top of the computer monitor a lay there or jump up onto the window sills to lie. Her and Sam where both fixed a few months ago.
Saturday brought a dramatic change in Abby. She violently attacked Sam and chased her until we could separate them. We place Abby in the bathroom to calm down. She would be hunched down with constant head movement as she expected a fight or danger on the other side of the door. When I released her she was in an aggressive stance and occasionally I would hear a low pitch growl, but no attacks.
Yesterday all seemed better when I noticed something strange. Abby was walking in front of me when she rolled on her right hip and jerked her head back and emitted an intense hiss and growl. Her right leg seemed immobilized and again she hissed and growled facing toward her rear and me. I turned to see if Sam was near and she was not. Abby has not shown any aggression to humans. I thought she may have had a muscle cramp. I since have stroked her near her back with no noticeable change in her disposition.
Last night Sam and Abby were fine together. I was rubbing Samâ€[emoji]8482[/emoji]s fur when I felt and removed a claw that seems to have completely of as if one ripped a finger nail off. It was luck as it didnâ€[emoji]8482[/emoji]t seem to penetrate Samâ€[emoji]8482[/emoji]s skin. Then this morning due their morning feed Abby once again attacked and pursued Sam until we could restrain her. Sam was so traumatized that she left a trail of urine through the house and I later saw a few small specks of blood on the bottom of one of her back legs. I checked and could not find any real injury on Sam.
In retrospect, since Abby has been fixed there have been subtle changes that we didnâ€[emoji]8482[/emoji]t put together. She doesnâ€[emoji]8482[/emoji]t jump to the windows as often and it is hard to remember the last time Abby has been on the computer monitor. Her demand for attention seems to be lessening as well as the duration. We have found that lately we have asked each other where Abby was as she seems to have become more reclusive. As I say there has been no abnormal aggression towards human friends or family members, but the last 2 days she has treated off and on Sam as a mortal enemy versus my normal playmate. Sam is absolutely terrified.
We took Abby to the vet today and the vet said she had a viral infection. She is on medication, but the vet had no real answers on the sudden mood change. Can anyone help?

I would see how things go when the medication has had time to take effect. Did the vet say what virus she is suffering from? I would also have her hips checked in case one is not properly seated in it's socket. This would cause sudden intence pain if it slips out and in again.

The vet did not say what kind of viral infection. Everything I posted was told to the Vet. We are keeping them seperated. We did leave them out for a moment, but Sam immediately started hissing and running as she is terrified. I hope that does not prove as a catalist for Abby to remain aggressive. We have no choice but to wait and see. Thanks for the support!

I would suggest that Abby was responding to the pain she was in. Aggression is often a sign that a cat is in pain. Her actions suggest she might have hurt herself jumping and if she doesn't start mellowing soon you might consider running her in for an x-ray

For now, I would use two things a feliway comfort zone room diffuser and pure vanilla extract. Plug the comfort zone into the room where the two cats are most of the time.

Then take the vanilla extract and put a small dab on each cat several times a day, under their chins, between their shoulders and the base of their tails. The will help neutralize their scent.

I am new to some of the cat terminology as we are Lab lovers (our old Lab Sarah was mother to Abby, Sarah has since past). What is a feliway comfort zone room diffuser? And, Thanks as we are (oh, my gosh!) becoming deeply attached (as if we aren't already) to the "kids".