Suddenly aggressive Cat

Well, I'm taking care of my sister's Cat, Sidda Lee since she can't have more then one cat where she lives. Sidda Lee seems to have become quite attached to me, and refuses to be away from me for more then five or ten minutes.

Sure, she's had her little hiss-matches with our other cats, and such, but she's rarely had a problem with our Dog, Daisy. They've maybe had one or two little snarling matches, but that's about it.

Lately, however, Sidda's become really aggressive towards Daisy. She'll hear or see Daisy from my bed, and suddenly dart down from the bed and start hissing and chasing Daisy, which makes Daisy start snapping at Sidda. Sidda's also been a little more mean towards my other cats. She also puffs her fur up, which lets me know that it's not just play behavior. However, when around me, and when there are no other animals around, she just sleeps, meows a whole lot, and seems perfectly fine. Any reason for her suddenly aggressive behavior towards our dog?

Whenever a cat's behaviour changes it's time for a trip to the vet as cats are very good at hiding whatever is bothering them.

Rainbow is right, a sudden change in behaviour warrants a vet visit with a full blood panel being done.

It sounds like it may be a behaviour problem rather than a health problem.

I have some questions for you. How long have you had her? How many cats did your sister have and was she friendly with them or did she have a special cat friend? Have you looked after Sidda before while your sister was on vacation? I'm wondering if she's now exhibiting anxiety and frustration because she "wants to go home". Another possibility is that your other cats and especially the dog are causing her stress and she's acting out and especially directing her frustration and aggression toward the dog? You say she's "sleeping" and "meowing a lot". Sleeping a lot can be a sign of depression. Meowing can be her way of trying to attract your attention if she's looking at you, or mourning a loss. Is she freefed or given her meals at specific times? Does the dog eat her food? Cats can get cranky if they're hungry or their blood sugar is dropping. Is she meowing while looking out a window or wandering around the house or in another room? She could be missing her old home or her cat friend(s).

Princess, I have a few more questions:

How old is Sidda Lee?
Did your sister have Sidda from time she was a kitten?
Was she the alpha cat of your sister's home?
How long have you had Sidda?

Her aggressive behaviour may be her way of readusting to her new home. Generally speaking, cats don't like change. She may be jealous of the dog, particularly if you pay a lot of attention to Daisy in her presence. The fact that she is comfortable with you when the other cats and Daisy aren't around, and refuses to be away from you, it sounds like she is trying to establish the "pecking order" in her new situation, that of being bosscat in your household.