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?