Since you've had her for over 8 years, I guess grieving isn't an issue. My thought is she's getting hard of hearing and, is possibly having a difficult time seeing at night. I have two senior cats and I always leave a nighlight on in my living room so they can find their dishes and litterboxes. Helps me not trip over things in my "landmind of a livingroom".

I do think a vet appt is called for. She's elderly and could possible be developing medical issues. Good luck with this beautiful girl. Keep us posted!!