12 year old cat suddenly attacking 9 year old cat

I have some serious cat issues going on at the moment... My 9 year old cat Lulu injured her right hind leg a week ago (we assume she fell off something high but no one was home when this occurred)... The vet thinks she tore her knee ligament and she's on pain medications and supposed to be resting for quite a few weeks as her leg heals.

However, both yesterday and today, my 12 year old cat Callie (who has lived with Lulu for Lulu's whole life!), has begun quite aggressively attacking Lulu! Twice she has done it at night and we have sprung out of bed to separate them, and twice she's done it during the day. It doesn't seem to be brought on by anything... although does seem to occur around when Callie might be hungry (I feed them separately 3 times a day). It's not like play fighting as Callie gets quite worked up - poofed up tail and everything - similar to how she gets when she sees a stranger cat outside. 

Now I'm not sure what to do! They act completely fine with each other for the most part during the day, even sleeping with one another and cuddling, and then Callie will snap on Lulu! Obviously I don't want Lulu getting injured more or making her current injury worse, and I'm even worried that maybe somehow Callie was the cause of Lulu's original injury? I have no idea what would make Callie start acting this way though. Nothing else has changed!

Any ideas / tips would be greatly appreciated

Update: The fighting continues 

I've been separating them at night and when no one's home so that they can't go balistic on each other with no one around to separate them.

The weird thing is that they can be in the same room together, they eat near each other, and I can bring one cat up to another and they sniff each other (sometimes even lay together sleeping!) and it's fine. But then randomly, Lulu will be sitting somewhere watching Callie (I think she's scared of her now), and Callie will start watching Lulu back... and then eventually Lulu will walk away and Callie will stalk Lulu as if she's about to attack! Then I of course try to grab them and separate them before a fight happens. But I don't understand this strange new behaviour!! 

I'm sorry you haven't had a reply yet.

Callie is going after Lulu because of her injury and very likely; her vet visit to go with it.  Cats know each other by scent and between the vet visit and the injury; Lulu doesn't smell like herself.  She smells like a completely different and unknown cat.  My 2 are litter mates and Noodles will hiss at Mooch for several days after a simple vet visit!  

I would separate them completely; especially so Lulu's leg can heal.  After a day or two to calm them; you can begin scent swapping to reintroduce them.  I would try a version of the same techniques you use when you introduce two new cats to each other.  Introducing cats is detailed in this article:
[article="32680"]How To Successfully Introduce Cats The Ultimate Guide  [/article]
There are some other articles linked at the bottom of this one that might have more helpful information on introductions and also on intervening in cat fights.  

I hope you are able to get things back to normal with them soon!!