What to do? Just bought a new mattress too, she seems to make a beeline for the bed. It's difficult to keep her downstairs and DC and DH find it impossible to keep bedroom door closed.
She's quite elderly and I've had her since she was 7 weeks old. She has hyperthyroidism and vet said one of her kidneys is enlarged. She's only started this behaviour recently.
I'm at the end of my tether now 
Any ideas?