Cat- Rear legs not holding her, not eating

Question
I noticed yesterday that my 18 year old cat hadn't gotten off her favorite bed all day, even to eat. She tried to stand, but became wobbly ob her rear legs & fell. I gave her water & she threw up. (She's always thrown up alot over the years.)I then put some karo syrup on her mouth to get some sugar into her system since she probably didn't eat all day. Outwardly she dosen't appear to be in pain & purrs when I rub her head. I stayed with her all night. My vet dosen't open till this afternoon & I'll try to get him to see her today. What could it be? She was fine up until I noticed her on the bed yesterday.

Answer
She may have had a stroke (blood clot) or it could be a combination of old age and arthritis.  Cats will also purr when they are in distress as it appears their purring soothes them.  Also, cats very often do not show pain because of their survival instinct.  If they are the one that is acting sick, they will be the one that is preyed upon.