Thank you everyone.

She had a blood test to see if it’s her kidneys, liver or thyroid. If it’s cancer the test won’t pick it up. She wandered around when she got home and did have something to eat. She doesn’t seem to react when I speak to her and when I picked her up and put her on my chest, she didn’t stay there. She curled up on the couch and had a sleep.
It was very scary. My son called me in a panic; she was on the floor, curled up but twisted at the same time and jerking violently. It went on for a few minutes, then she stopped and was making a weird meowing noise. She was salivating everywhere, then when she got up, she seemed not to have the use of her back legs for a bit. Then she started walking around. I had called the vet whilst she was fitting, so took her straight down. I don’t think she’s going to be with us for much longer.
Hubby dismantled the cat tree in case she has another seizure and falls off it.