Hi zubar,sorry to hear that,haven't been through this with any of my cats, but recently my friend has with hers.
Her cat was 20 years old and looking great up to about 6 months ago. She was thinking of having her put to sleep, but then she rallied round and was eating fine, although she was skinny and her fur had got that old cat "clumpy" look.
She disappeared 2 weeks ago and despite searching all the nearby gardens there was no trace of her :(
It's very sad, but yes it seems that they sometimes do this.