Need help with my cat. expert answers only plz?

Ok so my cat has been acting strange lately and it actually has me a little worried. I've had her since she was a weened kitten and she just turned 6 this month. She has always been a very social loving friendly cat loves attention that sort of thing. But lately it seems like she is losing her mind, literally. Nothing has changed about her home enviroment except my step daughter has started visiting more ( but as I said, my cat loves the attention) her liter box hasn't moved I haven't changed her diet nothing. So here's how she's acting weird. She's started sleeping sitting up. I know I know before you say it that's not the weird part. What is weird is while she's sleeping like this she just falls suddenly. She has fallen off the counter where her dishes are (which she never used to sleep near her food, another weird thing) she's fallen off the couch, the bannister railing she likes to sit on. Also, for some reason she started pooping in my fireplace. She won't pee in it just poop and she started peeing on the couch. Her liter box is still in its same place and still cleaned once a day just like it has been for 6 years. Also she isn't as social as she once was. When people come in she just waltzes to my room and hides till there gone, including when my husband comes in. She isn't as talkative as she used to be. All in all her overall demeanor is different (which I know can come with age) but what about the other odd things? Also she started kind of walking to her side. She just had a vet check up last month and was utd on her shots but this just started happening this week. Any thoughts?