Bipolar cat??

I have a 4 yr old Siamese spayed female who has started exhibiting some strange behavior. During the day she's lovey and purry and happy. At night she starts to growl and hiss for no seeming reason. It's like she becomes uber sensitive. She'll sit in your lap and just softly growl. She'll prompt you to scratch her head and then hiss at you when you do it. If you try to pick her up by her scruff she'll scratch the living crap out of you. It's like she's Jekyll and Hyde. Perfectly fine during the day......sensitive monster at night. I have no idea what's going on. There have been no changes in environment and she's in good health. Could this be a mental issue?

Are there any stray/feral cats around?  If so, maybe they are hanging around at night, disturbing her. Do you live in the city or country?

Not that I know of. I live in the country but few strays come around. And even when they do they don't come that close to the house.

This evening she was hiding in my closet and I thought I heard her fighting with my other cat. I went to break it up and passed my other cat sitting peacefully on a box. Becca was fighting with herself. But it really looked like she saw something in front of her. This is starting to freak me out. I'm really concerned about her. I have no idea what's going on,

Has she been vet checked? I'd do that first. Any changes in behavior in a cat is usually a sign something is going on.

On a side note, older cats should not be picked up by their scruff. It's ok in kittens cause they have the extra skin meant for such a thing as it's how their mom will pick them up. As they age they lose that extra skin and you could hurt her if it is done incorrectly. If you have to pick her up by the scruff (in cases where you are saving the cat from something that could hurt them) then make sure you support the cats weight with the other hand under their feet.
 

I would get her to the vet for a work up. Let us know what they say.